Display device

ABSTRACT

A display device with less light leakage and excellent contrast is provided. A display device having a high aperture ratio and including a large-capacitance capacitor is provided. A display device in which wiring delay due to parasitic capacitance is reduced is provided. A display device includes a transistor over a substrate, a pixel electrode connected to the transistor, a signal line electrically connected to the transistor, a scan line electrically connected to the transistor and intersecting with the signal line, and a common electrode overlapping with the pixel electrode and the signal line with an insulating film provided therebetween. The common electrode includes stripe regions extending in a direction intersecting with the signal line.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, oneembodiment of the present invention relates to a semiconductor device, adisplay device, a light-emitting device, a power storage device, adriving method thereof, or a manufacturing method thereof. Specifically,one embodiment of the present invention relates to a display device anda manufacturing method thereof.

2. Description of the Related Art

In recent years, liquid crystal has been used for a variety of devices;in particular, a liquid crystal display device (liquid crystal display)having features of thinness and lightness has been used for displays ina wide range of fields.

As a method for applying an electric field to a liquid crystal moleculeincluded in a liquid crystal display device, a vertical electric fieldmode and a horizontal electric field mode can be given. As a horizontalelectric field mode of a liquid crystal display panel, there are anin-plane switching (IPS) mode in which a pixel electrode and a commonelectrode are formed on the same insulating film and a fringe fieldswitching (FFS) mode in which a pixel electrode and a common electrodeoverlap with each other with an insulating film provided therebetween.

A liquid crystal display device of an FFS mode has a slit-shaped openingportion in a pixel electrode, and alignment of liquid crystal moleculesis controlled by applying an electric field generated between the pixelelectrode and a common electrode in the opening portion to the liquidcrystal molecules.

The liquid crystal display device of an FFS mode has a high apertureratio, a wide viewing angle, and an effect of improving an imagecontrast, and has been widely used recently (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2000-89255

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide adisplay device in which wiring delay due to parasitic capacitance isreduced. Another object of one embodiment of the present invention is toprovide a display device with little light leakage and excellentcontrast. Another object of one embodiment of the present invention isto provide a display device having a high aperture ratio and including alarge-capacitance capacitor. Another object of one embodiment of thepresent invention is to provide a display device with reduced powerconsumption. Another object of one embodiment of the present inventionis to provide a display device including a transistor having excellentelectrical characteristics. Another object of one embodiment of thepresent invention is to provide a novel display device. Another objectof one embodiment of the present invention is to provide a method formanufacturing a display device having a high aperture ratio and a wideviewing angle in fewer steps. Another object of one embodiment of thepresent invention is to provide a novel method for manufacturing adisplay device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a display device including atransistor over an insulating surface, a pixel electrode connected tothe transistor, a signal line connected to the transistor, a scan lineconnected to the transistor and intersecting with the signal line, and acommon electrode provided over the pixel electrode and the signal linewith an insulating film provided therebetween. The common electrodeincludes stripe regions extending in a direction intersecting with thesignal line.

The transistor includes a gate electrode electrically connected to thescan line, a semiconductor film overlapping with the gate electrode, agate insulating film between the gate electrode and the semiconductorfilm, a first conductive film electrically connected to the signal lineand the semiconductor film, and a second conductive film electricallyconnected to the pixel electrode and the semiconductor film. The secondconductive film includes a region parallel to the scan line and thestripe regions of the common electrode.

One embodiment of the present invention is a display device including,over an insulating surface, a signal line, a scan line, a transistor, apixel electrode, a common electrode, and a capacitor. The transistorincludes a gate electrode electrically connected to the scan line, asemiconductor film overlapping with the gate electrode, a gateinsulating film between the gate electrode and the semiconductor film, afirst conductive film electrically connected to the signal line and thesemiconductor film, and a second conductive film electrically connectedto the pixel electrode and the semiconductor film. The capacitorincludes the pixel electrode, the common electrode, and a nitrideinsulating film provided between the pixel electrode and the commonelectrode. The common electrode includes stripe regions extending in adirection intersecting with the signal line.

The second conductive film includes a region parallel to the scan lineand the stripe regions of the common electrode.

Each of the stripe regions of the common electrode may extend across aplurality of pixel electrodes provided parallel to the scan line.

An angle at which the common electrode and the signal line intersectwith each other is preferably larger than or equal to 70° and smallerthan or equal to 110°.

The pixel electrodes are provided in a matrix. The common electrodeincludes a region which intersects with the scan line and is connectedto the stripe regions. The semiconductor film and the pixel electrodeare in contact with the gate insulating film.

The semiconductor film and the pixel electrode each include an In—Gaoxide, an In—Zn oxide, or an In-M-Zn oxide (M represents Al, Ga, Y, Zr,La, Ce, or Nd).

The semiconductor film and the pixel electrode each have a multilayerstructure including a first film and a second film. An atomic ratio ofmetal elements of the first film is different from that of the secondfilm.

According to one embodiment of the present invention, a display devicein which wiring delay due to parasitic capacitance is reduced can beprovided. A display device with little light leakage and excellentcontrast can be provided. A display device having a high aperture ratioand including a large-capacitance capacitor can be provided. A displaydevice with reduced power consumption can be provided. A display deviceincluding a transistor having excellent electrical characteristics canbe provided. A display device having a high aperture ratio and a wideviewing angle can be manufactured in fewer steps.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a cross-sectional view and a top view illustratingone embodiment of a display device;

FIGS. 2A to 2D are top views illustrating embodiments of a displaydevice;

FIGS. 3A and 3B are a block diagram and a circuit diagram illustratingone embodiment of a display device;

FIG. 4 is a top view illustrating one embodiment of a display device;

FIG. 5 is a cross-sectional view illustrating one embodiment of atransistor;

FIGS. 6A to 6D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor;

FIGS. 7A to 7D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor;

FIGS. 8A to 8C are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor;

FIGS. 9A and 9B are a top view and a cross-sectional view illustratingone embodiment of a display device;

FIG. 10 is a top view illustrating one embodiment of a display device;

FIG. 11 is a top view illustrating one embodiment of a display device;

FIG. 12 is a cross-sectional view illustrating one embodiment of atransistor;

FIGS. 13A to 13C are cross-sectional views illustrating one embodimentof a method for manufacturing a transistor;

FIGS. 14A and 14B are cross-sectional views each illustrating oneembodiment of a transistor;

FIG. 15 illustrates a display module;

FIGS. 16A to 16D are each an external view of an electronic appliance ofan embodiment;

FIGS. 17A to 17D are top views of Sample 1 and Sample 2 and diagramsshowing transmittance distribution thereof;

FIGS. 18A to 18D are top views of Sample 3 and Sample 4 and diagramsshowing transmittance distribution thereof;

FIG. 19 is a top view illustrating one embodiment of a display device;

FIG. 20 is a cross-sectional view illustrating one embodiment of atransistor;

FIG. 21 is a cross-sectional view illustrating one embodiment of atransistor;

FIG. 22 is a cross-sectional view illustrating one embodiment of atransistor;

FIG. 23 is a cross-sectional view illustrating one embodiment of atransistor;

FIG. 24 is a top view illustrating one embodiment of a display device;

FIG. 25 is a top view illustrating one embodiment of a display device;and

FIG. 26 is a graph showing temperature dependence of conductivity.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that the mode and details can be variouslychanged without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments andexamples. In addition, in the following embodiments and examples, thesame portions or portions having similar functions are denoted by thesame reference numerals or the same hatching patterns in differentdrawings, and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through an“object having any electric function”. There is no particular limitationon an “object having any electric function” as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”are a switching element such as a transistor, a resistor, an inductor, acapacitor, and elements with a variety of functions as well as anelectrode and a wiring.

Embodiment 1

In this embodiment, a display device which is one embodiment of thepresent invention is described with reference to drawings.

FIG. 1A is a cross-sectional view of an FFS mode liquid crystal displaydevice and FIG. 1B is a top view of a pixel 10 in a display portionincluded in the liquid crystal display device. FIG. 1A corresponds to across-sectional view taken along dashed-dotted line A-B in FIG. 1B. InFIG. 1B, a substrate 1, an insulating film 3, an insulating film 8, aninsulating film 60, a substrate 61, a light-blocking film 62, a coloringfilm 63, an insulating film 64, an insulating film 65, and a liquidcrystal layer 66 are omitted.

As illustrated in FIGS. 1A and 1B, the FFS mode liquid crystal displaydevice is an active matrix liquid crystal display device and includes atransistor 102 and a pixel electrode 7 in each pixel 10 provided in thedisplay portion.

As illustrated in FIG. 1A, the liquid crystal display device includesthe transistor 102 over the substrate 1, the pixel electrode 7 connectedto the transistor 102, the insulating film 8 in contact with thetransistor 102 and the pixel electrode 7, a common electrode 9 incontact with the insulating film 8, and the insulating film 60 which isin contact with the insulating film 8 and the common electrode 9 andfunctions as an alignment film.

In addition, the light-blocking film 62 and the coloring film 63 whichare in contact with the substrate 61, the insulating film 64 in contactwith the substrate 61, the light-blocking film 62, and the coloring film63, and the insulating film 65 which is in contact with the insulatingfilm 64 and functions as an alignment film are provided. The liquidcrystal layer 66 is provided between the insulating film 60 and theinsulating film 65. Note that although not illustrated, a polarizingplate is provided outside each of the substrate 1 and the substrate 61.

The transistor 102 can be a staggered transistor, an inverted staggeredtransistor, a coplanar transistor, or the like as appropriate. In thecase of an inverted staggered transistor, a channel-etched structure, achannel protective structure, or the like can be used as appropriate.

The transistor 102 in this embodiment is an inverted staggeredtransistor having a channel-etched structure. The transistor 102includes a conductive film 2 functioning as a gate electrode over thesubstrate 1, the insulating film 3 functioning as a gate insulating filmover the substrate 1 and the conductive film 2, a semiconductor film 4overlapping with the conductive film 2 with the insulating film 3provided therebetween, and a conductive film 5 and a conductive film 6both in contact with the semiconductor film 4. The conductive film 2functions as a scan line as well as functions as a gate electrode. Inother words, the gate electrode is part of the scan line. The conductivefilm 5 functions as a signal line. The conductive films 5 and 6 functionas a source electrode and a drain electrode. In other words, one of thesource electrode and the drain electrode is part of the signal line.Accordingly, the transistor 102 is electrically connected to the scanline and the signal line. Although the conductive film 2 functions asthe gate electrode and the scan line here, the gate electrode and thescan line may be separately formed. The conductive film 5 functions asboth the signal line and the one of the source electrode and the drainelectrode, but the signal line and the one of the source electrode andthe drain electrode may be separately formed.

In the transistor 102, a semiconductor material such as silicon, silicongermanium, or an oxide semiconductor can be used for the semiconductorfilm 4 as appropriate. The semiconductor film 4 can have an amorphousstructure, a microcrystalline structure, a polycrystalline structure, asingle crystalline structure, or the like as appropriate.

As illustrated in FIG. 1B, the pixel electrode 7 is rectangular in thepixel 10. Since the display device of this embodiment is an activematrix liquid crystal display device, the pixel electrodes 7 are placedin a matrix. The pixel electrode 7 and the common electrode 9 are eachformed using a film having a light-transmitting property.

The shape of the pixel electrode 7 is not limited to a rectangularshape, and can be various shapes in accordance with the shape of thepixel 10. It is preferable that the pixel electrode 7 be widely formedin a region surrounded by the conductive film 2 functioning as a scanline and the conductive film 5 functioning as a signal line in the pixel10. Thus, the aperture ratio of the pixel 10 can be increased.

The common electrode 9 includes a plurality of regions (first regions)extending in a direction intersecting with the conductive film 5functioning as a signal line. That is, the common electrode 9 includesstripe regions (a plurality of first regions) extending in a directionintersecting with the conductive film 5 functioning as a signal line.The stripe regions are connected to a region (second region) extendingin a direction parallel or substantially parallel to the conductive film5 functioning as a signal line. That is, the common electrode 9 includesthe stripe regions (the plurality of first regions) and the connectionregion (second region) connected to the stripe regions.

In other words, the common electrode 9 includes, over the pixelelectrode 7, the plurality of regions (first regions) extending in adirection parallel or substantially parallel to the conductive film 2functioning as a scan line. That is, the common electrode 9 includes thestripe regions (the plurality of first regions) extending in a directionparallel or substantially parallel to the conductive film 2 functioningas a scan line. The stripe regions are connected to the region (secondregion) extending in a direction intersecting with the conductive film 2functioning as a scan line.

An angle at which a direction in which the stripe regions (the pluralityof first regions) of the common electrode 9 extend and a direction inwhich the conductive film 5 functioning as a signal line extendsintersect with each other is preferably larger than or equal to 70° andsmaller than or equal to 110°. When the two directions intersect witheach other at the angle in the above range, light leakage can bereduced. Further, since the common electrode 9 is not formed over theentire surface of the substrate 1 but includes the stripe regions (theplurality of first regions), parasitic capacitance generated between thecommon electrode 9 and the conductive film 2 functioning as a scan lineand between the common electrode 9 and the conductive film 5 functioningas a signal line can be reduced.

The stripe regions (the plurality of first regions) of the commonelectrode 9 can each have a linear shape. Alternatively, the striperegions (the plurality of first regions) of the common electrode 9 mayeach have a zigzag shape or a wavy shape. In the case where the striperegions (the plurality of first regions) of the common electrode 9 eachhave a zigzag shape or a wavy shape, multi-domain alignment of liquidcrystal molecules is obtained, and thus the viewing angle can beimproved.

Because of the stripe shape of the common electrode 9, a parabolicelectric field is generated between the pixel electrode 7 and the commonelectrode 9 as indicated by dashed arrows in FIG. 1A when voltage isapplied to the pixel electrode 7. Accordingly, liquid crystal moleculesincluded in the liquid crystal layer 66 can be aligned.

In a region where the pixel electrode 7 and the common electrode 9overlap with each other, the pixel electrode 7, the insulating film 8,and the common electrode 9 form a capacitor. Since the pixel electrode 7and the common electrode 9 are each formed using a film having alight-transmitting property, the aperture ratio and the capacitance ofthe capacitor can be increased. Furthermore, when the insulating film 8provided between the pixel electrode 7 and the common electrode 9 isformed using a material having a high dielectric constant, a largeamount of charges can be accumulated in the capacitor. As the materialhaving a high dielectric constant, silicon nitride, aluminum oxide,gallium oxide, yttrium oxide, hafnium oxide, hafnium silicate(HfSiO_(x)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), or the like can be given.

The light-blocking film 62 functions as a black matrix. The coloringfilm 63 functions as a color filter. The coloring film 63 is notnecessarily provided in the case where the liquid crystal display deviceis a monochrome display device, for example.

The coloring film 63 is a coloring film that transmits light in aspecific wavelength range. For example, a red (R) film for transmittinglight in a red wavelength range, a green (G) film for transmitting lightin a green wavelength range, a blue (B) film for transmitting light in ablue wavelength range, or the like can be used.

The light-blocking film 62 preferably has a function of blocking lightin a specific wavelength range, and can be a metal film, an organicinsulating film including a black pigment, or the like.

The insulating film 65 functions as a planarization layer or suppressesdiffusion of impurities in the coloring film 63 to the liquid crystalelement side.

Although not illustrated, a sealant is provided between the substrate 1and the substrate 61. The liquid crystal layer 66 is enclosed by thesubstrate 1, the substrate 61, and the sealant. A spacer for keeping thethickness of the liquid crystal layer 66 (also referred to as a cellgap) may be provided between the insulating film 60 and the insulatingfilm 64.

Next, a method for driving the liquid crystal display device of thisembodiment is described with reference to FIGS. 2A to 2D.

FIGS. 2A to 2D are each a top view of pixels included in the pixelportion of the FFS mode liquid crystal display device. In each of FIGS.2A to 2D, two adjacent pixels 10 a and 10 b are shown. In each of FIGS.2A and 2B, the common electrode 9 extends in a direction parallel orsubstantially parallel to the conductive film 2 functioning as a scanline. In other words, the common electrode 9 is laid across the pixels10 a and 10 b.

In each of FIGS. 2A and 2B, the pixels 10 a and 10 b are provided withthe common electrode 9 including stripe regions extending in a directionintersecting with conductive films 5 a and 5 b functioning as a signalline. In each of FIGS. 2C and 2D, the pixels 10 a and 10 b are providedwith the common electrode 9 including stripe regions extending in adirection intersecting with the conductive film 2 functioning as a scanline. A method for driving a display element in a pixel, in which blackdisplay in an initial state is turned into white display by applicationof voltage to a pixel electrode, i.e., a method for driving a displayelement of a normally black mode, is described. Note that a displayelement here is the pixel electrode 7, the common electrode 9, and aliquid crystal molecule included in the liquid crystal layer. Although amethod for driving a display element of a normally black mode isdescribed in this embodiment, a method for driving a display element ofa normally white mode can be used as appropriate.

In the case of black display, voltage at which a transistor is turned onis applied to a scan line, and 0 V is applied to a signal line and acommon electrode. As a result, 0 V is applied to the pixel electrode. Inother words, an electric field is not generated between the pixelelectrode and the common electrode, and thus liquid crystal molecules donot operate.

In the case of white display, voltage at which a transistor is turned onis applied to a scan line, voltage at which liquid crystal moleculesoperate, e.g., 6 V, is applied to a signal line, and 0 V is applied to acommon electrode. As a result, 6 V is applied to the pixel electrode. Inother words, an electric field is generated between the pixel electrodeand the common electrode, and thus the liquid crystal molecules operate.

Here, a negative liquid crystal material is used in this description.The liquid crystal molecules are aligned in a direction perpendicular tothe common electrode in an initial state. The alignment of the liquidcrystal molecules in an initial state is referred to as initialalignment. The liquid crystal molecules rotate in a plane parallel to asubstrate by application of voltage between the pixel electrode and thecommon electrode. Although the negative liquid crystal material is usedin this embodiment, a positive liquid crystal material can be used asappropriate.

The polarizing plate is provided outside each of the substrate 1 and thesubstrate 61 in FIG. 1A. A polarizer of the polarizing plate providedoutside the substrate 1 and a polarizer of the polarizing plate providedoutside the substrate 61 are placed to intersect with each other atright angles, that is, placed in a crossed Nicols state. Therefore, whenthe liquid crystal molecules are aligned in a direction parallel to theconductive film 2 functioning as a scan line or the conductive films 5 aand 5 b functioning as a signal line, light is absorbed by thepolarizing plates and black is displayed. Although the polarizers areplaced in a crossed Nicols state in this embodiment, the polarizers canbe placed in a parallel Nicols state as appropriate.

In each of FIGS. 2A to 2D, the pixel 10 a includes the conductive film 2functioning as a scan line, a semiconductor film 4 a, a conductive film5 a functioning as a signal line, a conductive film 6 a, a pixelelectrode 7 a, and the common electrode 9, and the pixel 10 b includesthe conductive film 2 functioning as a scan line, a semiconductor film 4b, a conductive film 5 b functioning as a signal line, a conductive film6 b, a pixel electrode 7 b, and the common electrode 9. FIGS. 2A and 2Ceach illustrate an initial state and FIGS. 2B and 2D each illustrate astate where the pixel 10 b performs white display.

Since the common electrode 9 included in the pixels 10 a and 10 b ineach of FIGS. 2C and 2D extends in a direction parallel or substantiallyparallel to the conductive films 5 a and 5 b functioning as signallines, liquid crystal molecules L are aligned in a directionperpendicular to the conductive films 5 a and 5 b functioning as signallines in an initial state (black display) illustrated in FIG. 2C.

The case where the pixel 10 a performs black display and the pixel 10 bperforms white display, as in FIG. 2D, is described. To the commonelectrode 9 and the conductive film 5 a functioning as a signal line isapplied 0 V. To the conductive film 5 b functioning as a signal line isapplied 6 V. As a result, 6 V is applied to the pixel electrode 7 b inthe pixel 10 b, an electric field as indicated by arrows in FIG. 2D isgenerated between the pixel electrode 7 b and the common electrode 9,and the liquid crystal molecules L are aligned accordingly. Here, theliquid crystal molecules L rotate by 45°.

A potential of the pixel electrode 7 a is 0 V in the pixel 10 a and apotential of the conductive film 5 b functioning as a signal line, whichis provided in the vicinity of the pixel electrode 7 a, is 6 V.Therefore, also in the pixel 10 a, an electric field as indicated by anarrow in FIG. 2D is generated between the pixel electrode 7 a and theconductive film 5 b functioning as a signal line, and the liquid crystalmolecules L are aligned accordingly. As a result, in the pixel 10 awhere black display should be performed, alignment of some of the liquidcrystal molecules L is changed, causing light leakage.

In contrast, in the pixels 10 a and 10 b in each of FIGS. 2A and 2B, thecommon electrode 9 extends in a direction perpendicular to theconductive films 5 a and 5 b functioning as signal lines; therefore, theliquid crystal molecules L are aligned in a direction parallel orsubstantially parallel to the conductive films 5 a and 5 b functioningas signal lines in an initial state (black display).

The case where the pixel 10 a performs black display and the pixel 10 bperforms white display, as in FIG. 2B, is described. To the commonelectrode 9 and the conductive film 5 a functioning as a signal line isapplied 0 V. To the conductive film 5 b functioning as a signal line isapplied 6 V. As a result, 6 V is applied to the pixel electrode 7 b inthe pixel 10 b, an electric field as indicated by arrows in FIG. 2B isgenerated between the pixel electrode 7 b and the common electrode 9,and the liquid crystal molecules L are aligned accordingly. Here, theliquid crystal molecules L rotate by −45°.

The potential of the pixel electrode 7 a is 0 V in the pixel 10 a andthe potential of the conductive film 5 b functioning as a signal line,which is provided in the vicinity of the pixel electrode 7 a, is 6 V.However, since the common electrode 9 and the conductive film 5 bfunctioning as a signal line intersect with each other, a first electricfield F1 generated between the pixel electrode 7 a and the conductivefilm 5 b functioning as a signal line and a major axis of the liquidcrystal molecule L intersect with each other at right angles. As aresult, the liquid crystal molecule L, which is included in a negativeliquid crystal material, does not operate and thus light leakage can besuppressed.

For the above-described reason, when a common electrode extending in adirection intersecting with a signal line is provided in an FFS modeliquid crystal display device, the display device can have excellentcontrast.

The common electrode 9 of this embodiment is not formed over the entiresurface of the substrate. Therefore, a region where the common electrode9 overlaps with the conductive films 5 a and 5 b functioning as signallines can be reduced and thus parasitic capacitance generated betweenthe signal line and the common electrode 9 can be reduced. As a result,wiring delay can be reduced in a display device formed using a largesubstrate.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a display device which is one embodiment of thepresent invention is described with reference to drawings. In addition,in this embodiment, an oxide semiconductor film is used as asemiconductor film included in a transistor.

A display device illustrated in FIG. 3A includes a pixel portion 101; ascan line driver circuit 104; a signal line driver circuit 106; m scanlines 107 which are arranged parallel or substantially parallel to eachother and whose potentials are controlled by the scan line drivercircuit 104; and n signal lines 109 which are arranged parallel orsubstantially parallel to each other and whose potentials are controlledby the signal line driver circuit 106. Furthermore, the pixel portion101 includes a plurality of pixels 103 arranged in a matrix.Furthermore, common lines 115 arranged parallel or substantiallyparallel to each other are provided along the signal lines 109. The scanline driver circuit 104 and the signal line driver circuit 106 arecollectively referred to as a driver circuit portion in some cases.

Each scan line 107 is electrically connected to the n pixels 103 in thecorresponding row among the pixels 103 arranged in m rows and n columnsin the pixel portion 101. Each signal line 109 is electrically connectedto the m pixels 103 in the corresponding column among the pixels 103arranged in in rows and n columns. Note that in and n are each aninteger of 1 or more. Each common line 115 is electrically connected tothe m pixels 103 in the corresponding column among the pixels 103arranged in m rows and n columns.

FIG. 3B illustrates an example of a circuit configuration that can beused for the pixels 103 in the display device illustrated in FIG. 3A.

The pixel 103 in FIG. 3B includes a liquid crystal element 121, atransistor 102, and a capacitor 105.

One of a pair of electrodes of the liquid crystal element 121 isconnected to the transistor 102 and the potential thereof is setaccording to the specifications of the pixel 103 as appropriate. Theother of the pair of electrodes of the liquid crystal element 121 isconnected to the common line 115 and a common potential is appliedthereto. The alignment of liquid crystal molecules of the liquid crystalelement 121 is controlled in accordance with data written to thetransistor 102.

The liquid crystal element 121 is an element that controls transmissionor non-transmission of light utilizing an optical modulation action of aliquid crystal molecule. Note that the optical modulation action of theliquid crystal molecule is controlled by an electric field applied tothe liquid crystal molecule (including a horizontal electric field, avertical electric field, and a diagonal electric field). Examples of aliquid crystal material used for the liquid crystal element 121 are anematic liquid crystal, a cholesteric liquid crystal, a smectic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, aferroelectric liquid crystal, and an anti-ferroelectric liquid crystal.

An FFS mode is used as a method for driving the display device includingthe liquid crystal element 121.

The liquid crystal element may be formed using a liquid crystalcomposition including a liquid crystal material exhibiting a blue phaseand a chiral material. The liquid crystal exhibiting a blue phase has ashort response time of 1 msec or less and is optically isotropic;therefore, alignment treatment is not necessary and viewing angledependence is small.

In the structure of the pixel 103 illustrated in FIG. 3B, one of asource electrode and a drain electrode of the transistor 102 iselectrically connected to the signal line 109, and the other iselectrically connected to the one of the pair of electrodes of theliquid crystal element 121. A gate electrode of the transistor 102 iselectrically connected to the scan line 107. The transistor 102 has afunction of controlling whether to write a data signal by being turnedon or off.

In the pixel 103 in FIG. 3B, one of a pair of electrodes of thecapacitor 105 is connected to the transistor 102. The other of the pairof electrodes of the capacitor 105 is electrically connected to thecommon line 115. The potential of the common line 115 is set inaccordance with the specifications of the pixel 103 as appropriate. Thecapacitor 105 functions as a storage capacitor for storing written data.In this embodiment, the one of the pair of electrodes of the capacitor105 is the one of the pair of electrodes of the liquid crystal element121. The other of the pair of electrodes of the capacitor 105 is theother of the pair of electrodes of the liquid crystal element 121.

A specific structure of an element substrate included in the displaydevice is described. FIG. 4 is a top view of a plurality of pixels 103a, 103 b, and 103 c.

In FIG. 4 , a conductive film 13 functioning as a scan line extends in adirection substantially perpendicularly to the signal line (in thehorizontal direction in the drawing). A conductive film 21 a functioningas a signal line extends in a direction substantially perpendicularly tothe scan line (in the vertical direction in the drawing). Note that theconductive film 13 functioning as a scan line is electrically connectedto the scan line driver circuit 104 (see FIG. 3A), and the conductivefilm 21 a functioning as a signal line is electrically connected to thesignal line driver circuit 106 (see FIG. 3A).

The transistor 102 is provided at a region where the scan line and thesignal line intersect with each other. The transistor 102 includes theconductive film 13 functioning as a gate electrode; a gate insulatingfilm (not illustrated in FIG. 4 ); an oxide semiconductor film 19 awhere a channel region is formed, over the gate insulating film; and theconductive film 21 a and a conductive film 21 b functioning as a sourceelectrode and a drain electrode. The conductive film 13 also functionsas a scan line, and a region of the conductive film 13 that overlapswith the oxide semiconductor film 19 a serves as the gate electrode ofthe transistor 102. In addition, the conductive film 21 a also functionsas a signal line, and a region of the conductive film 21 a that overlapswith the oxide semiconductor film 19 a functions as the source electrodeor the drain electrode of the transistor 102. Furthermore, in the topview of FIG. 4 , an end portion of the scan line is located on the outerside of an end portion of the oxide semiconductor film 19 a. Thus, thescan line functions as a light-blocking film for blocking light from alight source such as a backlight. For this reason, the oxidesemiconductor film 19 a included in the transistor is not irradiatedwith light, so that a variation in the electrical characteristics of thetransistor can be suppressed.

The conductive film 21 b is electrically connected to the pixelelectrode 19 b. A common electrode 29 is provided over the pixelelectrode 19 b with an insulating film provided therebetween. An openingportion 40 indicated by a dashed-dotted line is provided in theinsulating film provided over the pixel electrode 19 b. The pixelelectrode 19 b is in contact with a nitride insulating film (notillustrated in FIG. 4 ) in the opening portion 40.

The common electrode 29 includes stripe regions (a plurality of firstregions) extending in a direction intersecting with a signal line. Theplurality of first regions is connected to a second region extending ina direction parallel or substantially parallel to a signal line.Accordingly, the stripe regions (the plurality of first regions) of thecommon electrode 29 are at the same potential.

The capacitor 105 is formed in a region where the pixel electrode 19 band the common electrode 29 overlap with each other. The pixel electrode19 b and the common electrode 29 each have a light-transmittingproperty. That is, the capacitor 105 has a light-transmitting property.

As illustrated in FIG. 4 , the liquid crystal display device describedin this embodiment is an FFS mode liquid crystal display device and isprovided with the common electrode 29 including the stripe regionsextending in a direction intersecting with a signal line. Thus, thedisplay device can have excellent contrast.

Owing to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (in a large area) in the pixel 103.Thus, a display device with a large-capacitance capacitor as well as anaperture ratio increased to typically 50% or more, preferably 60% ormore can be provided. For example, in a high-resolution display devicesuch as a liquid crystal display device, the area of a pixel is smalland accordingly the area of a capacitor is also small. For this reason,the amount of charges accumulated in the capacitor is small in thehigh-resolution display device. However, since the capacitor 105 of thisembodiment has a light-transmitting property, when the capacitor 105 isprovided in a pixel, enough capacitance can be obtained in the pixel andthe aperture ratio can be improved. Typically, the capacitor 105 can befavorably used for a high-resolution display device with a pixel densityof 200 pixels per inch (ppi) or more, 300 ppi or more, or furthermore,500 ppi or more.

In a liquid crystal display device, as the capacitance value of acapacitor is increased, a period during which the alignment of liquidcrystal molecules of a liquid crystal element can be kept constant inthe state where an electric field is applied can be made longer. Whenthe period can be made longer in a display device which displays a stillimage, the number of times of rewriting image data can be reduced,leading to a reduction in power consumption. Further, according to thestructure of this embodiment, the aperture ratio can be improved even ina high-resolution display device, which makes it possible to use lightfrom a light source such as a backlight efficiently, so that powerconsumption of the display device can be reduced.

Note that a top view of one embodiment of the present invention is notlimited to FIG. 4 . The display device can have a variety of differentstructures. For example, connection regions of the common electrode 29may be formed over conductive films functioning as signal lines asillustrated in FIG. 19 .

Next, FIG. 5 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 4 . The transistor 102 shown in FIG. 5 is achannel-etched transistor. Note that the transistor 102 in the channellength direction and the capacitor 105 are illustrated in thecross-sectional view taken along dashed-dotted line A-B, and thetransistor 102 in the channel width direction is illustrated in thecross-sectional view taken along dashed-dotted line C-D.

The transistor 102 in FIG. 5 has a single-gate structure and includesthe conductive film 13 functioning as a gate electrode over thesubstrate 11. In addition, the transistor 102 includes a nitrideinsulating film 15 formed over the substrate 11 and the conductive film13 functioning as a gate electrode, an oxide insulating film 17 formedover the nitride insulating film 15, the oxide semiconductor film 19 aoverlapping with the conductive film 13 functioning as a gate electrodewith the nitride insulating film 15 and the oxide insulating film 17provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode which are incontact with the oxide semiconductor film 19 a. Moreover, an oxideinsulating film 23 is formed over the oxide insulating film 17, theoxide semiconductor film 19 a, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode, and an oxideinsulating film 25 is formed over the oxide insulating film 23. Anitride insulating film 27 is formed over the oxide insulating film 23,the oxide insulating film 25, and the conductive film 21 b. The pixelelectrode 19 b is formed over the oxide insulating film 17. The pixelelectrode 19 b is connected to one of the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode, here, connectedto the conductive film 21 b. The common electrode 29 is formed over thenitride insulating film 27.

A region where the pixel electrode 19 b, the nitride insulating film 27,and the common electrode 29 overlap with one another functions as thecapacitor 105.

Note that a cross-sectional view of one embodiment of the presentinvention is not limited to FIG. 5 . The display device can have avariety of different structures. For example, the pixel electrode 19 bmay have a slit. The pixel electrode 19 b may have a comb-like shape. Anexample of a cross-sectional view in this case is shown in FIG. 20 .Alternatively, an insulating film 26 b may be provided over the nitrideinsulating film 27 as illustrated in FIG. 21 . For example, an organicresin film may be provided as the insulating film 26 b. Thus, theinsulating film 26 b can have a flat surface. In other words, as anexample, the insulating film 26 b can function as a planarization film.Alternatively, a capacitor 105 b may be formed so that the commonelectrode 29 and the conductive film 21 b overlap with each other.Examples of a cross-sectional view in this case are shown in FIG. 22 andFIG. 23 .

A structure of the display device is described below in detail.

There is no particular limitation on the property of a material and thelike of the substrate 11 as long as the material has heat resistanceenough to withstand at least later heat treatment. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 11. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOI(silicon on insulator) substrate, or the like may be used as thesubstrate 11. Furthermore, any of these substrates further provided witha semiconductor element may be used as the substrate 11. In the casewhere a glass substrate is used as the substrate 11, a glass substratehaving any of the following sizes can be used: the 6th generation (1500mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation(2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10thgeneration (2950 mm×3400 mm). Thus, a large-sized display device can bemanufactured.

Alternatively, a flexible substrate may be used as the substrate 11, andthe transistor 102 may be provided directly on the flexible substrate.Alternatively, a separation layer may be provided between the substrate11 and the transistor 102. The separation layer can be used when part orthe whole of a display device formed over the separation layer isseparated from the substrate 11 and transferred onto another substrate.In such a case, the transistor 102 can be transferred to a substratehaving low heat resistance or a flexible substrate as well.

The conductive film 13 functioning as a gate electrode can be formedusing a metal element selected from aluminum, chromium, copper,tantalum, titanium, molybdenum, and tungsten; an alloy containing any ofthese metal elements as a component; an alloy containing any of thesemetal elements in combination; or the like. Further, one or more metalelements selected from manganese and zirconium may be used. Theconductive film 13 functioning as a gate electrode may have asingle-layer structure or a stacked structure of two or more layers. Forexample, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which an aluminum film is stacked overa titanium film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a titanium nitride film, a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film, a two-layer structure in which a copperfilm is stacked over a titanium film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, an alloy film or anitride film which contains aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The conductive film 13 functioning as a gate electrode can also beformed using a light-transmitting conductive material such as indium tinoxide, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added. It is also possible to have astacked structure formed using the above light-transmitting conductivematerial and the above metal element.

The nitride insulating film 15 can be a nitride insulating film that ishardly permeated by oxygen. Furthermore, a nitride insulating film thatis hardly permeated by oxygen, hydrogen, and water can be used. As thenitride insulating film that is hardly permeated by oxygen and thenitride insulating film that is hardly permeated by oxygen, hydrogen,and water, a silicon nitride film, a silicon nitride oxide film, analuminum nitride film, an aluminum nitride oxide film, or the like isgiven. Instead of the nitride insulating film that is hardly permeatedby oxygen and the nitride insulating film that is hardly permeated byoxygen, hydrogen, and water, an oxide insulating film such as analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, or a hafnium oxynitride film can be used.

The thickness of the nitride insulating film 15 is preferably greaterthan or equal to 5 nm and less than or equal to 100 nm, more preferablygreater than or equal to 20 nm and less than or equal to 80 nm.

The oxide insulating film 17 may be formed to have a single-layerstructure or a stacked structure using, for example, one or more of asilicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, an aluminum oxide film, a hafnium oxide film, a gallium oxidefilm, a Ga—Zn-based metal oxide film, and a silicon nitride film.

The oxide insulating film 17 may also be formed using a material havinga high relative dielectric constant such as hafnium silicate(HfSiO_(x)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the oxide insulating film 17 is preferably greater thanor equal to 5 nm and less than or equal to 400 nm, more preferablygreater than or equal to 10 nm and less than or equal to 300 nm, furtherpreferably greater than or equal to 50 nm and less than or equal to 250nm.

The oxide semiconductor film 19 a is typically formed using In—Ga oxide,In—Zn oxide, or In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, orNd).

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are preferably as follows: the atomic percentage ofIn is greater than 25 atomic % and the atomic percentage of M is lessthan 75 atomic %, or more preferably, the atomic percentage of In isgreater than 34 atomic % and the atomic percentage of M is less than 66atomic %.

The energy gap of the oxide semiconductor film 19 a is 2 eV or more,preferably 2.5 eV or more, more preferably 3 eV or more. The off-statecurrent of the transistor 102 can be reduced by using an oxidesemiconductor having such a wide energy gap.

The thickness of the oxide semiconductor film 19 a is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, more preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, La, Ce, or Nd), it is preferable thatthe atomic ratio of metal elements of a sputtering target used forforming the In-M-Zn oxide film satisfy In≥M and Zn≥M. As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that theproportion of each metal element in the atomic ratio of the oxidesemiconductor film 19 a to be formed varies within a range of ±40% ofthat in the above atomic ratio of the sputtering target as an error.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 19 a. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, preferably 1×10¹⁵/cm³ orlower, more preferably 1×10¹³/cm³ or lower, much more preferably1×10¹¹/cm³ or lower is used as the oxide semiconductor film 19 a.

Note that, without limitation to the compositions and materialsdescribed above, a material with an appropriate composition may be useddepending on required semiconductor characteristics and electricalcharacteristics (e.g., field-effect mobility and threshold voltage) of atransistor. Further, in order to obtain required semiconductorcharacteristics of a transistor, it is preferable that the carrierdensity, the impurity concentration, the defect density, the atomicratio of a metal element to oxygen, the interatomic distance, thedensity, and the like of the oxide semiconductor film 19 a be set to beappropriate.

Note that it is preferable to use, as the oxide semiconductor film 19 a,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (theamount of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor in which a channelregion is formed in the oxide semiconductor film rarely has a negativethreshold voltage (is rarely normally on). A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasa low density of defect states and accordingly has few carrier traps insome cases. Further, the highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶ μmand a channel length (L) of 10 μm, the off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drainvoltage) between a source electrode and a drain electrode of from 1 V to10 V. Thus, the transistor in which a channel region is formed in theoxide semiconductor film has a small variation in electricalcharacteristics and high reliability in some cases. As examples of theimpurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, andthe like are given.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Further, in some cases, bonding of part of hydrogen to oxygen bonded toa metal element causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductor film19 a. Specifically, in the oxide semiconductor film 19 a, theconcentration of hydrogen which is measured by secondary ion massspectrometry (SIMS) is set to lower than or equal to 5×10¹⁹ atoms/cm³,preferably lower than or equal to 1×10¹⁹ atoms/cm³, more preferablylower than or equal to 5×10¹⁸ atoms/cm³, still more preferably lowerthan or equal to 1×10¹⁸ atoms/cm³, yet more preferably lower than orequal to 5×10¹⁷ atoms/cm³, further preferably lower than or equal to1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 19 a, oxygen vacancies areincreased in the oxide semiconductor film 19 a, and the oxidesemiconductor film 19 a becomes an n-type film. Thus, the concentrationof silicon or carbon (the concentration is measured by SIMS) of theoxide semiconductor film 19 a is set to lower than or equal to 2×10¹⁸atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

The concentration of alkali metal or alkaline earth metal in the oxidesemiconductor film 19 a, which is measured by SIMS, is set to lower thanor equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶atoms/cm³. Alkali metal and alkaline earth metal might generate carrierswhen bonded to an oxide semiconductor, in which case the off-statecurrent of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal in the oxide semiconductor film 19 a.

Further, when containing nitrogen, the oxide semiconductor film 19 aeasily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenwhich is measured by SIMS is preferably set to, for example, lower thanor equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 19 a may have a non-single-crystalstructure, for example. The non-single-crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure which is described later, or an amorphous structure, forexample. Among the non-single-crystal structures, the amorphousstructure has the highest density of defect states, whereas CAAC-OS hasthe lowest density of defect states.

The oxide semiconductor film 19 a may have an amorphous structure, forexample. An oxide semiconductor film having an amorphous structure hasdisordered atomic arrangement and no crystalline component, for example.

Note that the oxide semiconductor film 19 a may be a mixed filmincluding two or more of the following: a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Further, the mixed film has astacked-layer structure of two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases.

The pixel electrode 19 b is formed by processing an oxide semiconductorfilm formed at the same time as the oxide semiconductor film 19 a. Thus,the pixel electrode 19 b contains a metal element similar to that in theoxide semiconductor film 19 a. Further, the pixel electrode 19 b has acrystal structure similar to or different from that of the oxidesemiconductor film 19 a. By adding impurities or oxygen vacancies to theoxide semiconductor film formed at the same time as the oxidesemiconductor film 19 a, the oxide semiconductor film has conductivityand thus functions as the pixel electrode 19 b. An example of theimpurities contained in the oxide semiconductor film is hydrogen.Instead of hydrogen, as the impurity, boron, phosphorus, tin, antimony,a rare gas element, an alkali metal, an alkaline earth metal, or thelike may be included. Alternatively, the pixel electrode 19 b is formedat the same time as the oxide semiconductor film 19 a, and has increasedconductivity by containing oxygen vacancies generated by plasma damageor the like. Alternatively, the pixel electrode 19 b is formed at thesame time as the oxide semiconductor film 19 a, and has increasedconductivity by containing impurities and oxygen vacancies generated byplasma damage or the like.

The oxide semiconductor film 19 a and the pixel electrode 19 b are bothformed over the oxide insulating film 17, but differ in impurityconcentration. Specifically, the pixel electrode 19 b has a higherimpurity concentration than the oxide semiconductor film 19 a. Forexample, the concentration of hydrogen contained in the oxidesemiconductor film 19 a is lower than or equal to 5×10¹⁹ atoms/cm³,preferably lower than or equal to 1×10¹⁹ atoms/cm³, more preferablylower than or equal to 5×10¹⁸ atoms/cm³, still more preferably lowerthan or equal to 1×10¹⁸ atoms/cm³, yet more preferably lower than orequal to 5×10¹⁷ atoms/cm³, further preferably lower than or equal to1×10¹⁶ atoms/cm³. The concentration of hydrogen contained in the pixelelectrode 19 b is higher than or equal to 8×10¹⁹ atoms/cm³, preferablyhigher than or equal to 1×10²⁰ atoms/cm³, more preferably higher than orequal to 5×10²⁰ atoms/cm³. The concentration of hydrogen contained inthe pixel electrode 19 b is greater than or equal to 2 times, preferablygreater than or equal to 10 times that in the oxide semiconductor film19 a.

When the oxide semiconductor film formed at the same time as the oxidesemiconductor film 19 a is exposed to plasma, the oxide semiconductorfilm is damaged, and oxygen vacancies can be generated. For example,when a film is formed over the oxide semiconductor film by a plasma CVDmethod or a sputtering method, the oxide semiconductor film is exposedto plasma and oxygen vacancies are generated. Alternatively, when theoxide semiconductor film is exposed to plasma in etching treatment forformation of the oxide insulating film 23 and the oxide insulating film25, oxygen vacancies are generated. Alternatively, when the oxidesemiconductor film is exposed to plasma of a mixed gas of oxygen andhydrogen, hydrogen, a rare gas, ammonia, or the like, oxygen vacanciesare generated. As a result, the conductivity of the oxide semiconductorfilm is increased, so that the oxide semiconductor film functions as thepixel electrode 19 b.

In other words, the pixel electrode 19 b is formed using an oxidesemiconductor film having high conductivity. It can also be said thatthe pixel electrode 19 b is formed using a metal oxide film having highconductivity.

In the case where a silicon nitride film is used as the nitrideinsulating film 27, the silicon nitride film contains hydrogen. Whenhydrogen in the nitride insulating film 27 is diffused into the oxidesemiconductor film formed at the same time as the oxide semiconductorfilm 19 a, hydrogen is bonded to oxygen and electrons serving ascarriers are generated in the oxide semiconductor film. When the siliconnitride film is formed by a plasma CVD method or a sputtering method,the oxide semiconductor film is exposed to plasma and oxygen vacanciesare generated in the oxide semiconductor film. When hydrogen containedin the silicon nitride film enters the oxygen vacancies, electronsserving as carriers are generated. As a result, the conductivity of theoxide semiconductor film is increased, so that the oxide semiconductorfilm functions as the pixel electrode 19 b.

When hydrogen is added to an oxide semiconductor including oxygenvacancies, hydrogen enters oxygen vacant sites and forms a donor levelin the vicinity of the conduction band. As a result, the conductivity ofthe oxide semiconductor is increased, so that the oxide semiconductorbecomes a conductor. An oxide semiconductor having become a conductorcan be referred to as an oxide conductor. In other words, the pixelelectrode 19 b is formed using an oxide conductor Oxide semiconductorsgenerally have a visible light-transmitting property because of theirlarge energy gap. An oxide conductor is an oxide semiconductor having adonor level in the vicinity of the conduction band. Therefore, theinfluence of absorption due to the donor level is small, and an oxideconductor has a visible light transmitting property comparable to thatof an oxide semiconductor.

The pixel electrode 19 b has lower resistivity than the oxidesemiconductor film 19 a. The resistivity of the pixel electrode 19 b ispreferably greater than or equal to 1×10⁻⁸ times and less than 1×10⁻¹times the resistivity of the oxide semiconductor film 19 a. Theresistivity of the pixel electrode 19 b is typically greater than orequal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, preferably greater than orequal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

The conductive films 21 a and 21 b functioning as a source electrode anda drain electrode are each formed to have a single-layer structure or astacked-layer structure including any of metals such as aluminum,titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum,silver, tantalum, and tungsten or an alloy containing any of thesemetals as its main component. For example, a single-layer structure ofan aluminum film containing silicon, a two-layer structure in which analuminum film is stacked over a titanium film, a two-layer structure inwhich an aluminum film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, a three-layer structure in whicha molybdenum film or a molybdenum nitride film, an aluminum film or acopper film, and a molybdenum film or a molybdenum nitride film arestacked in this order, and the like can be given. Note that atransparent conductive material containing indium oxide, tin oxide, orzinc oxide may be used.

As the oxide insulating film 23 or the oxide insulating film 25, anoxide insulating film which contains more oxygen than that in thestoichiometric composition is preferably used. Here, as the oxideinsulating film 23, an oxide insulating film which is permeated byoxygen is formed, and as the oxide insulating film 25, an oxideinsulating film which contains more oxygen than that in thestoichiometric composition is formed.

The oxide insulating film 23 is an oxide insulating film which ispermeated by oxygen. Thus, oxygen released from the oxide insulatingfilm 25 provided over the oxide insulating film 23 can be moved to theoxide semiconductor film 19 a through the oxide insulating film 23.Moreover, the oxide insulating film 23 also functions as a film thatrelieves damage to the oxide semiconductor film 19 a at the time offorming the oxide insulating film 25 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the oxide insulating film 23. Note that in thisspecification, “silicon oxynitride film” refers to a film that containsoxygen at a higher proportion than nitrogen, and “silicon nitride oxidefilm” refers to a film that contains nitrogen at a higher proportionthan oxygen.

Further, it is preferable that the amount of defects in the oxideinsulating film 23 be small and typically, the spin density of a signalthat appears at g=2.001 be lower than or equal to 3×10¹⁷ spins/cm³ byelectron spin resonance (ESR) measurement. The signal that appears atg=2.001 is due to dangling bonds of silicon. This is because if thedensity of defects in the oxide insulating film 23 is high, oxygen isbonded to the defects and the amount of oxygen that passes through theoxide insulating film 23 is decreased.

Further, it is preferable that the amount of defects at the interfacebetween the oxide insulating film 23 and the oxide semiconductor film 19a be small and typically, the spin density of a signal that appears atg=1.93 due to an oxygen vacancy in the oxide semiconductor film 19 a belower than or equal to 1×10¹⁷ spins/cm³, more preferably lower than orequal to the lower limit of detection by ESR measurement.

Note that in the oxide insulating film 23, all oxygen that enters theoxide insulating film 23 from the outside is transferred to the outsideof the oxide insulating film 23 in some cases. Alternatively, someoxygen that enters the oxide insulating film 23 from the outside remainsin the oxide insulating film 23 in some cases. Further, movement ofoxygen occurs in the oxide insulating film 23 in some cases in such amanner that oxygen enters the oxide insulating film 23 from the outsideand oxygen contained in the oxide insulating film 23 is transferred tothe outside of the oxide insulating film 23.

The oxide insulating film 25 is formed in contact with the oxideinsulating film 23. The oxide insulating film 25 is formed using anoxide insulating film which contains oxygen at a higher proportion thanthe stoichiometric composition. Part of oxygen is released by heatingfrom the oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition. The oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of thefilm surface in the TDS analysis is preferably higher than or equal to100° C. and lower than or equal to 700° C., or higher than or equal to100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the oxide insulating film 25.

It is preferable that the amount of defects in the oxide insulating film25 be small and typically, the spin density of a signal that appears atg=2.001 be lower than 1.5×10¹⁸ spins/cm³, more preferably lower than orequal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxideinsulating film 25 is provided more apart from the oxide semiconductorfilm 19 a than the oxide insulating film 23 is; thus, the oxideinsulating film 25 may have higher defect density than the oxideinsulating film 23.

Like the nitride insulating film 15, the nitride insulating film 27 canbe a nitride insulating film which is hardly permeated by oxygen.Furthermore, a nitride insulating film which is hardly permeated byoxygen, hydrogen, and water can be used.

The nitride insulating film 27 is formed using a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like with a thickness greater than or equalto 50 nm and less than or equal to 300 nm, preferably greater than orequal to 100 nm and less than or equal to 200 nm.

In the case where the oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition is included in theoxide insulating film 23 or the oxide insulating film 25, part of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 can be transferred to the oxide semiconductor film 19 a, so that theamount of oxygen vacancies contained in the oxide semiconductor film 19a can be reduced.

The threshold voltage of a transistor using an oxide semiconductor filmwith oxygen vacancies easily shifts negatively, and such a transistortends to be normally on. This is because charges are generated owing tooxygen vacancies in the oxide semiconductor film and the resistance isthus reduced. The transistor having normally-on characteristics causesvarious problems in that malfunction is likely to be caused when inoperation and that power consumption is increased when not in operation,for example. Further, there is a problem in that the amount of change inelectrical characteristics, typically in threshold voltage, of thetransistor is increased by change over time or a stress test.

However, in the transistor 102 in this embodiment, the oxide insulatingfilm 23 or the oxide insulating film 25 provided over the oxidesemiconductor film 19 a contains oxygen at a higher proportion than thestoichiometric composition. As a result, oxygen contained in the oxideinsulating film 23 or the oxide insulating film 25 is moved to the oxidesemiconductor film 19 a efficiently, so that the amount of oxygenvacancies in the oxide semiconductor film 19 a can be reduced.Accordingly, a transistor having normally-off characteristics isobtained. Further, the amount of change in electrical characteristics,typically in threshold voltage, of the transistor over time or due to astress test can be reduced.

The common electrode 29 is formed using a light-transmitting conductivefilm. As the light-transmitting conductive film, an indium oxide filmcontaining tungsten oxide, an indium zinc oxide film containing tungstenoxide, an indium oxide film containing titanium oxide, an indium tinoxide film containing titanium oxide, an indium tin oxide (hereinafter,referred to as ITO) film, an indium zinc oxide film, an indium tin oxidefilm to which silicon oxide is added, and the like are given.

The common electrode 29 includes the stripe regions extending in adirection intersecting with the conductive film 21 a functioning as asignal line. Accordingly, in the vicinity of the pixel electrode 19 band the conductive film 21 a, unintended alignment of liquid crystalmolecules can be prevented and thus light leakage can be suppressed. Asa result, a display device with excellent contrast can be manufactured.

On an element substrate of the display device described in thisembodiment, the pixel electrode is formed at the same time as the oxidesemiconductor film of the transistor. The pixel electrode also functionsas one of electrodes of the capacitor. The common electrode alsofunctions as the other of electrodes of the capacitor. Thus, a step offorming another conductive film is not needed to form the capacitor, andthe number of steps of manufacturing the display device can be reduced.The capacitor has a light-transmitting property. As a result, the areaoccupied by the capacitor can be increased and the aperture ratio in apixel can be increased.

Next, a method for manufacturing the transistor 102 and the capacitor105 in FIG. 5 is described with reference to FIGS. 6A to 6D, FIGS. 7A to7D, and FIGS. 8A to 8C.

As illustrated in FIG. 6A, a conductive film 12 to be the conductivefilm 13 is formed over the substrate 11. The conductive film 12 isformed by a sputtering method, a chemical vapor deposition (CVD) methodsuch as a metal organic chemical vapor deposition (MOCVD) method, ametal chemical vapor deposition method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method,an evaporation method, a pulsed laser deposition (PLD) method, or thelike. When a metal organic chemical vapor deposition (MOCVD) method, ametal chemical vapor deposition method, or an atomic layer deposition(ALD) method is employed, the conductive film is less damaged by plasma.

Here, a glass substrate is used as the substrate 11. Further, as theconductive film 12, a 100-nm-thick tungsten film is formed by asputtering method.

Then, a mask is formed over the conductive film 12 by a photolithographyprocess using a first photomask. Next, as illustrated in FIG. 6B, partof the conductive film 12 is etched with the use of the mask to form theconductive film 13 functioning as a gate electrode. After that, the maskis removed.

Note that the conductive film 13 functioning as a gate electrode may beformed by an electrolytic plating method, a printing method, an ink jetmethod, or the like instead of the above formation method.

Here, the tungsten film is etched by dry etching to form the conductivefilm 13 functioning as a gate electrode.

Next, as illustrated in FIG. 6C, over the conductive film 13 functioningas a gate electrode, the nitride insulating film 15 and an oxideinsulating film 16 to be the oxide insulating film 17 later are formed.Then, over the oxide insulating film 16, an oxide semiconductor film 18to be the oxide semiconductor film 19 a and the pixel electrode 19 blater is formed.

The nitride insulating film 15 and the oxide insulating film 16 are eachformed by a sputtering method, a chemical vapor deposition (CVD) methodsuch as a metal organic chemical vapor deposition (MOCVD) method, ametal chemical vapor deposition method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method,an evaporation method, a pulsed laser deposition (PLD) method, a coatingmethod, a printing method, or the like. When a metal organic chemicalvapor deposition (MOCVD) method, a metal chemical vapor depositionmethod, or an atomic layer deposition (ALD) method is employed, thenitride insulating film 15 and the oxide insulating film 16 are lessdamaged by plasma. When an atomic layer deposition (ALD) method isemployed, coverage of the nitride insulating film 15 and the oxideinsulating film 16 can be increased.

Here, as the nitride insulating film 15, a 300-nm-thick silicon nitridefilm is formed by a plasma CVD method in which silane, nitrogen, andammonia are used as a source gas.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the oxide insulating film 16, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxidecan be given as examples.

Moreover, in the case of forming a gallium oxide film as the oxideinsulating film 16, an MOCVD method can be employed.

Here, as the oxide insulating film 16, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane and dinitrogenmonoxide are used as a source gas.

The oxide semiconductor film 18 can be formed by a sputtering method, achemical vapor deposition (CVD) method such as a metal organic chemicalvapor deposition (MOCVD) method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method, apulsed laser deposition method, a laser ablation method, a coatingmethod, or the like. When a metal organic chemical vapor deposition(MOCVD) method or an atomic layer deposition (ALD) method is employed,the oxide semiconductor film 18 is less damaged by plasma and the oxideinsulating film 16 is less damaged. When an atomic layer deposition(ALD) method is employed, coverage of the oxide semiconductor film 18can be increased.

As a power supply device for generating plasma in the case of formingthe oxide semiconductor film by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen to a rare gas is preferably increased.

Further, a target may be selected as appropriate in accordance with thecomposition of the oxide semiconductor film to be formed.

In order to obtain a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film, besides the high vacuumevacuation of the chamber, a high purification of a sputtering gas isalso needed. As an oxygen gas or an argon gas used for a sputtering gas,a gas which is highly purified to have a dew point of −40° C. or lower,preferably −80° C. or lower, further preferably −100° C. or lower, stillfurther preferably −120° C. or lower is used, whereby entry of moistureor the like into the oxide semiconductor film can be prevented as muchas possible.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide target(In:Ga:Zn=1:1:1).

Then, after a mask is formed over the oxide semiconductor film 18 by aphotolithography process using a second photomask, the oxidesemiconductor film is partly etched using the mask. Thus, the oxidesemiconductor film 19 a and an oxide semiconductor film 19 c subjectedto element isolation as illustrated in FIG. 6D are formed. After that,the mask is removed.

Here, the oxide semiconductor films 19 a and 19 c are formed in such amanner that a mask is formed over the oxide semiconductor film 18 andpart of the oxide semiconductor film 18 is selectively etched by a wetetching method.

Next, as illustrated in FIG. 7A, a conductive film 20 to be theconductive films 21 a and 21 b later is formed.

The conductive film 20 can be formed by a method similar to that of theconductive film 12 as appropriate.

Here, a 50-nm-thick tungsten film and a 300-nm-thick copper film aresequentially stacked by a sputtering method.

Next, a mask is formed over the conductive film 20 by a photolithographyprocess using a third photomask. Then, the conductive film 20 is etchedusing the mask, so that the conductive films 21 a and 21 b functioningas a source electrode and a drain electrode are formed as illustrated inFIG. 7B. After that, the mask is removed.

Here, a mask is formed over the copper film by a photolithographyprocess. Then, the tungsten film and the copper film are etched with theuse of the mask, so that the conductive films 21 a and 21 b are formed.Note that the copper film is etched by a wet etching method. Next, thetungsten film is etched by a dry etching method using SF₆, wherebyfluoride is formed on the surface of the copper film. By the fluoride,diffusion of copper elements from the copper film is reduced and thusthe copper concentration in the oxide semiconductor film 19 a can bereduced.

Next, as illustrated in FIG. 7C, an oxide insulating film 22 to be theoxide insulating film 23 later and an oxide insulating film 24 to be theoxide insulating film 25 later are formed over the oxide semiconductorfilms 19 a and 19 c and the conductive films 21 a and 21 b. The oxideinsulating film 22 and the oxide insulating film 24 can each be formedby a method similar to those of the nitride insulating film 15 and theoxide insulating film 16 as appropriate.

Note that after the oxide insulating film 22 is formed, the oxideinsulating film 24 is preferably formed in succession without exposureto the air. After the oxide insulating film 22 is formed, the oxideinsulating film 24 is formed in succession by adjusting at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature without exposure to the air, whereby theconcentration of impurities attributed to the atmospheric component atthe interface between the oxide insulating film 22 and the oxideinsulating film 24 can be reduced and oxygen in the oxide insulatingfilm 24 can be moved to the oxide semiconductor film 19 a; accordingly,the amount of oxygen vacancies in the oxide semiconductor film 19 a canbe reduced.

As the oxide insulating film 22, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure is greater thanor equal to 20 Pa and less than or equal to 250 Pa, preferably greaterthan or equal to 100 Pa and less than or equal to 250 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power is supplied to an electrode provided in thetreatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 22. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

With the use of the above conditions, an oxide insulating film which ispermeated by oxygen can be formed as the oxide insulating film 22.Further, by providing the oxide insulating film 22, damage to the oxidesemiconductor film 19 a can be reduced in a step of forming the oxideinsulating film 25 which is formed later.

Under the above film formation conditions, the bonding strength ofsilicon and oxygen becomes strong in the above substrate temperaturerange. Thus, as the oxide insulating film 22, a dense and hard oxideinsulating film which is permeated by oxygen, typically, a silicon oxidefilm or a silicon oxynitride film having an etching rate lower than orequal to 10 nm/min, preferably lower than or equal to 8 nm/min whenetching is performed at 25° C. using hydrofluoric acid of 0.5 wt % canbe formed.

The oxide insulating film 22 is formed while heating is performed; thus,hydrogen, water, or the like contained in the oxide semiconductor film19 a can be released in the step. Hydrogen contained in the oxidesemiconductor film 19 a is bonded to an oxygen radical formed in plasmato form water. Since the substrate is heated in the step of forming theoxide insulating film 22, water formed by bonding of oxygen and hydrogenis released from the oxide semiconductor film. That is, when the oxideinsulating film 22 is formed by a plasma CVD method, the amount of waterand hydrogen contained in the oxide semiconductor film 19 a can bereduced.

Further, time for heating in a state where the oxide semiconductor film19 a is exposed can be shortened because heating is performed in a stepof forming the oxide insulating film 22. Thus, the amount of oxygenreleased from the oxide semiconductor film by heat treatment can bereduced. That is, the amount of oxygen vacancies in the oxidesemiconductor film can be reduced.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the oxide insulating film 22 can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorfilm 19 a can be reduced; thus, the negative shift in the thresholdvoltage of the transistor can be inhibited.

Here, as the oxide insulating film 22, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 30 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as a source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 150W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film which is permeated by oxygen can be formed.

As the oxide insulating film 24, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 24. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

As the film formation conditions of the oxide insulating film 24, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the oxide insulating film 24 becomes higher than thatin the stoichiometric composition. On the other hand, in the film formedat a substrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Further, the oxide insulatingfilm 22 is provided over the oxide semiconductor film 19 a. Accordingly,in the step of forming the oxide insulating film 24, the oxideinsulating film 22 functions as a protective film of the oxidesemiconductor film 19 a. Consequently, the oxide insulating film 24 canbe formed using the high-frequency power having a high power densitywhile damage to the oxide semiconductor film 19 a is reduced.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as the source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 1500W is supplied to the parallel-plate electrodes with the use of a 27.12MHz high-frequency power source. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 0.25 W/cm².

Further, when the conductive films 21 a and 21 b functioning as a sourceelectrode and a drain electrode is formed, the oxide semiconductor film19 a is damaged by the etching of the conductive film, so that oxygenvacancies are generated on the back channel side of the oxidesemiconductor film 19 a (the side of the oxide semiconductor film 19 awhich is opposite to the side facing to the conductive film 13functioning as a gate electrode). However, with the use of the oxideinsulating film which contains oxygen at a higher proportion than thestoichiometric composition as the oxide insulating film 24, the oxygenvacancies generated on the back channel side can be repaired by heattreatment. By this, defects contained in the oxide semiconductor film 19a can be reduced, and thus, the reliability of the transistor 102 can beimproved.

Then, a mask is formed over the oxide insulating film 24 by aphotolithography process using a fourth photomask. Next, as illustratedin FIG. 7D, part of the oxide insulating film 22 and part of the oxideinsulating film 24 are etched with the use of the mask to form the oxideinsulating film 23 and the oxide insulating film 25 having the openingportion 40. After that, the mask is removed.

In the process, the oxide insulating films 22 and 24 are preferablyetched by a dry etching method. As a result, the oxide semiconductorfilm 19 c is exposed to plasma in the etching treatment; thus, theamount of oxygen vacancies in the oxide semiconductor film 19 c can beincreased.

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., more preferably higher than or equal to320° C. and lower than or equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 25 can be moved to the oxide semiconductor film 19 a, so that theamount of oxygen vacancies contained in the oxide semiconductor film 19a can be reduced.

In the case where water, hydrogen, or the like is contained in the oxideinsulating film 23 and the oxide insulating film 25 and the nitrideinsulating film 26 has a barrier property against water, hydrogen, orthe like, when the nitride insulating film 26 is formed later and heattreatment is performed, water, hydrogen, or the like contained in theoxide insulating film 23 and the oxide insulating film 25 are moved tothe oxide semiconductor film 19 a, so that defects are generated in theoxide semiconductor film 19 a. However, by the heating, water, hydrogen,or the like contained in the oxide insulating film 23 and the oxideinsulating film 25 can be released; thus, variation in electricalcharacteristics of the transistor 102 can be reduced, and change inthreshold voltage can be inhibited.

Note that when the oxide insulating film 24 is formed over the oxideinsulating film 22 while being heated, oxygen can be moved to the oxidesemiconductor film 19 a to reduce the amount of oxygen vacancies in theoxide semiconductor film 19 a; thus, the heat treatment is notnecessarily performed.

The heat treatment may be performed after the formation of the oxideinsulating films 22 and 24. However, the heat treatment is preferablyperformed after the formation of the oxide insulating films 23 and 25because a film having higher conductivity can be formed in such a mannerthat oxygen is not moved to the oxide semiconductor film 19 c and oxygenis released from the oxide semiconductor film 19 c because of exposureof the oxide semiconductor film 19 c and then oxygen vacancies aregenerated.

Here, the heat treatment is performed at 350° C. in an atmosphere ofnitrogen and oxygen for one hour.

Then, as illustrated in FIG. 8A, the nitride insulating film 26 isformed.

The nitride insulating film 26 can be formed by a method similar tothose of the nitride insulating film 15 and the oxide insulating film 16as appropriate. By forming the nitride insulating film 26 by asputtering method, a CVD method, or the like, the oxide semiconductorfilm 19 c is exposed to plasma; thus, the amount of oxygen vacancies inthe oxide semiconductor film 19 c can be increased.

The oxide semiconductor film 19 c has improved conductivity andfunctions as the pixel electrode 19 b. When a silicon nitride film isformed by a plasma CVD method as the nitride insulating film 26,hydrogen contained in the silicon nitride film is diffused into theoxide semiconductor film 19 c; thus, the conductivity of the pixelelectrode 19 b can be enhanced.

In the case where a silicon nitride film is formed by a plasma CVDmethod as the nitride insulating film 26, the substrate placed in thetreatment chamber of the plasma CVD apparatus that is vacuum-evacuatedis preferably held at a temperature higher than or equal to 300° C. andlower than or equal to 400° C., more preferably higher than or equal to320° C. and lower than or equal to 370° C., so that a dense siliconnitride film can be formed.

In the case where a silicon nitride film is formed, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used as asource gas. As the source gas, a small amount of ammonia compared to theamount of nitrogen is used, whereby ammonia is dissociated in the plasmaand activated species are generated. The activated species cleave a bondbetween silicon and hydrogen which are contained in a deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which bondsbetween silicon and nitrogen are promoted and bonds between silicon andhydrogen is few, can be formed. On the other hand, when the amount ofammonia is larger than the amount of nitrogen in the source gas,cleavage of a deposition gas containing silicon and cleavage of nitrogenare not promoted, so that a sparse silicon nitride film in which bondsbetween silicon and hydrogen remain and defects are increased is formed.Therefore, in the source gas, the flow ratio of the nitrogen to theammonia is set to be preferably greater than or equal to 5 and less thanor equal to 50, more preferably greater than or equal to 10 and lessthan or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed as the nitride insulating film 26 by aplasma CVD method in which silane with a flow rate of 50 sccm, nitrogenwith a flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccmare used as the source gas, the pressure in the treatment chamber is 100Pa, the substrate temperature is 350° C., and a high-frequency power of1000 W is supplied to parallel-plate electrodes with a high-frequencypower supply of 27.12 MHz. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 1.7×10⁻¹ W/cm².

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., more preferably higher than or equal to320° C. and lower than or equal to 370° C. As a result, the negativeshift of the threshold voltage can be reduced. Moreover, the amount ofchange in the threshold voltage can be reduced.

Next, although not illustrated, a mask is formed over the nitrideinsulating film 26 by a photolithography process using a fifthphotomask, and the nitride insulating film 26 is etched using the mask.Thus, a conductive film formed at the same time as the conductive films21 a and 21 b is exposed and the nitride insulating film 27 is formed.The conductive film is connected to the common electrode 29 to be formedlater.

Next, as illustrated in FIG. 8B, a conductive film 28 to be the commonelectrode 29 later is formed over the nitride insulating film 27.

The conductive film 28 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 8C, partof the conductive film 28 is etched with the use of the mask to form thecommon electrode 29. Although not illustrated, the common electrode 29is connected to a connection terminal formed at the same time as theconductive film 13 or a connection terminal formed at the same time asthe conductive films 21 a and 21 b. After that, the mask is removed.

Through the above process, the transistor 102 is manufactured and thecapacitor 105 can also be manufactured.

The element substrate of the display device of this embodiment isprovided with a common electrode including stripe regions extending in adirection intersecting with a signal line. Therefore, the display devicecan have excellent contrast.

On the element substrate of the display device of this embodiment, thepixel electrode is formed at the same time as the oxide semiconductorfilm of the transistor; therefore, the transistor 102 and the capacitor105 can be formed using six photomasks. The pixel electrode functions asthe one of electrodes of the capacitor. The common electrode functionsas the other of electrodes of the capacitor. Thus, a step of forminganother conductive film is not needed to form the capacitor, and thenumber of steps of manufacturing the display device can be reduced. Thecapacitor has a light-transmitting property. As a result, the areaoccupied by the capacitor can be increased and the aperture ratio in apixel can be increased.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Modification Example 1

A structure in which a common line connected to the common electrode isprovided in the display device described in Embodiment 1 is describedwith reference to FIGS. 9A and 9B.

FIG. 9A is a top view illustrating the pixels 103 a, 103 b, and 103 cincluded in a display device, and FIG. 9B is a cross-sectional viewtaken along dashed-dotted lines A-B and C-D in FIG. 9A.

As illustrated in FIG. 9A, a common line 21 c extending in a directionparallel or substantially parallel to the conductive film 21 afunctioning as a signal line is formed. For easy understanding of thestructure of the common electrode 29, the common electrode 29 is hatchedin FIG. 9A to explain its shape. The common electrode 29 includes aplurality of first regions hatched with diagonally left down lines and asecond region hatched with diagonally right down lines. The plurality ofthe first regions is a plurality of stripe regions. The second regionextends in a direction parallel or substantially parallel to theconductive film 21 a functioning as a signal line. The second region canbe referred to as a connection region connecting to the plurality offirst regions (stripe regions). The common line 21 c overlaps with theconnection region (second region) of the common electrode 29.

The common line 21 c may be provided par pixel. Alternatively, thecommon line 21 c may be provided every plurality of pixels. For example,as illustrated in FIG. 9A, one common line 21 c is provided for everythree pixels, so that the area occupied by the common line in thedisplay device can be reduced. As a result, the area of the pixel andthe aperture ratio of the pixel can be increased.

In a region where the pixel electrode 19 b and the common electrode 29overlap with each other, a liquid crystal molecule is less likely to bedriven by an electric field generated between the pixel electrode 19 band the connection region (second region) of the common electrode 29.Therefore, the area of a region overlapping with the pixel electrode 19b in the connection region of the common electrode 29 is reduced, sothat a region where a liquid crystal molecule is driven can beincreased, leading to an increase in the aperture ratio. For example, asillustrated in FIG. 9A, the connection region of the common electrode 29is provided so as not to overlap with the pixel electrode 19 b, wherebythe area of a region where the pixel electrode 19 b and the commonelectrode 29 overlap with each other can be reduced and thus theaperture ratio of the pixel can be increased.

Although one common line 21 c is provided for the three pixels 103 a,103 b, and 103 c in FIG. 9A, one common line may be provided for everytwo pixels. Alternatively, one common line may be provided for everyfour or more pixels.

As illustrated in FIG. 9B, the common line 21 c can be formed at thesame time as the conductive film 21 a functioning as a signal line. Thecommon electrode 29 is connected to the common line 21 c in an openingportion 42 formed in the oxide insulating film 23, the oxide insulatingfilm 25, and the nitride insulating film 27.

Since a material of the conductive film 21 a has resistivity lower thanthat of the common electrode 29, resistance of the common electrode 29and the common line 21 c can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, a display device which is different from the displaydevice in Embodiment 2 and a manufacturing method thereof are describedwith reference to a drawing. This embodiment is different fromEmbodiment 2 in that a transistor included in a high resolution displaydevice includes a source electrode and a drain electrode capable ofreducing light leakage. Note that the description of the same structuresas those in Embodiment 2 is omitted.

FIG. 10 is a top view of a display device described in this embodiment.The conductive film 21 b functioning as one of a source electrode and adrain electrode has an L shape in the top view. In other words, theconductive film 21 b has a shape in which a region 21 b_1 extending in adirection perpendicular to the conductive film 13 functioning as a scanline and a region 21 b_2 extending in a direction parallel orsubstantially parallel to the conductive film 13 are connected to eachother in the top view. The region 21 b_2 overlaps with one or more ofthe conductive film 13, the pixel electrode 19 b, and the commonelectrode 29 in the top view. Alternatively, the conductive film 21 bincludes the region 21 b_2 extending in a direction parallel orsubstantially parallel to the conductive film 13 and the region 21 b_2is placed between the conductive film 13 and the pixel electrode 19 b orthe common electrode 29 in the top view.

Since the area of the pixel in a high resolution display device isreduced, the distance between the common electrode 29 and the conductivefilm 13 functioning as a scan line is reduced. In a pixel performingblack display, when voltage at which a transistor is turned on isapplied to the conductive film 13 functioning as a scan line, anelectric field is generated between the pixel electrode 19 b and theconductive film 13 functioning as a scan line. As a result, a liquidcrystal molecule rotates in an unintended direction, causing lightleakage.

However, in the transistor included in the display device of thisembodiment, the conductive film 21 b functioning as the one of a sourceelectrode and a drain electrode includes the region 21 b_2 overlappingwith one or more of the conductive film 13, the pixel electrode 19 b,and the common electrode 29, or the region 21 b_2 placed between theconductive film 13 and the pixel electrode 19 b or the common electrode29 in the top view. As a result, the region 21 b_2 blocks the electricfield of the conductive film 13 functioning as a scan line and anelectric field generated between the conductive film 13 and the pixelelectrode 19 b can be suppressed, leading to a reduction in lightleakage.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 4

In this embodiment, a display device which is different from the displaydevices in Embodiments 2 and 3 and a manufacturing method thereof aredescribed with reference to drawings. This embodiment is different fromEmbodiment 2 in that a high resolution display device includes a commonelectrode capable of reducing light leakage. Note that the descriptionof the same structures as those in Embodiment 2 is omitted.

FIG. 11 is a top view of a display device described in this embodiment.A common electrode 29 a includes stripe regions 29 a_1 extending in adirection intersecting with the conductive film 21 a functioning as asignal line and a region 29 a_2 which is connected to the stripe regionsand overlaps with the conductive film 13 functioning as a scan line.

Since the area of a pixel is reduced in a high resolution displaydevice, the distance between the pixel electrode 19 b and the conductivefilm 13 functioning as a scan line is reduced. When voltage is appliedto the conductive film 13 functioning as a scan line, an electric fieldis generated between the conductive film 13 and the pixel electrode 19b. As a result, a liquid crystal molecule rotates in an unintendeddirection, causing light leakage.

However, the display device described in this embodiment includes thecommon electrode 29 a including the region 29 a_2 intersecting with theconductive film 13 functioning as a scan line. Therefore, an electricfield can be prevented from being generated between the common electrode29 a and the conductive film 13 functioning as a scan line, leading to areduction in light leakage.

Note that a top view of one embodiment of the present invention is notlimited to FIG. 11 . The display device can have a variety of differentstructures. For example, as illustrated in FIG. 24 or FIG. 25 , thecommon electrode 29 a may include a region overlapping with part of theconductive film 13 functioning as a scan line. A channel region formedin the oxide semiconductor film 19 a of the transistor does not overlapwith the common electrode 29 a. Accordingly, an electric field of thecommon electrode 29 a is not applied to the channel region, resulting ina reduction in leakage current of the transistor. Furthermore, thecommon electrode 29 a in FIG. 25 includes a region overlapping with theconductive film 13 functioning as a scan line and the conductive film 21a functioning as a signal line. Therefore, electric fields of theconductive film 13 and the conductive film 21 a can be blocked by thecommon electrode 29 a, so that alignment disorder of liquid crystalmolecules can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 5

In this embodiment, a display device which is different from the displaydevice in Embodiment 2 and a manufacturing method thereof are describedwith reference to drawings. The display device in this embodiment isdifferent from that in Embodiment 2 in that the transistor has astructure in which an oxide semiconductor film is provided betweendifferent gate electrodes, that is, a dual-gate structure. Note that thedescription of the same structures as those in Embodiment 2 is omitted.

A specific structure of an element substrate included in the displaydevice is described. The element substrate in this embodiment isdifferent from that in Embodiment 2 in that a conductive film 29 bfunctioning as a gate electrode and overlapping part of or the whole ofeach of the conductive film 13 functioning as a gate electrode, theoxide semiconductor film 19 a, the conductive films 21 a and 21 b, andthe oxide insulating film 25 is provided as shown in FIG. 12 . Theconductive film 29 b functioning as a gate electrode is connected to theconductive film 13 functioning as a gate electrode in opening portions41 a and 41 b.

A transistor 102 a shown in FIG. 12 is a channel-etched transistor. Notethat a cross-sectional view along line A-B shows the transistor 102 a inthe channel length direction and a capacitor 105 a, and across-sectional view along line C-D shows the transistor 102 a in thechannel width direction and a connection portion between the conductivefilm 13 functioning as a gate electrode and the conductive film 29 bfunctioning as a gate electrode.

The transistor 102 a in FIG. 12 has a dual-gate structure and includesthe conductive film 13 functioning as a gate electrode over thesubstrate 11. In addition, the transistor 102 a includes the nitrideinsulating film 15 formed over the substrate 11 and the conductive film13 functioning as a gate electrode, the oxide insulating film 17 formedover the nitride insulating film 15, the oxide semiconductor film 19 aoverlapping with the conductive film 13 functioning as a gate electrodewith the nitride insulating film 15 and the oxide insulating film 17provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode which are incontact with the oxide semiconductor film 19 a. Moreover, the oxideinsulating film 23 is formed over the oxide insulating film 17, theoxide semiconductor film 19 a, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode, and the oxideinsulating film 25 is formed over the oxide insulating film 23. Thenitride insulating film 27 is formed over the nitride insulating film15, the oxide insulating film 23, the oxide insulating film 25, and theconductive film 21 b. The pixel electrode 19 b is formed over the oxideinsulating film 17. The pixel electrode 19 b is connected to one of theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, here, connected to the conductive film 21 b. The commonelectrode 29 and the conductive film 29 b functioning as a gateelectrode are formed over the nitride insulating film 27.

As illustrated in the cross-sectional view along line C-D, theconductive film 29 b functioning as a gate electrode is connected to theconductive film 13 functioning as a gate electrode in the openingportions 41 a and 41 b provided in the nitride insulating film 15 andthe nitride insulating film 27. That is, the conductive film 13functioning as a gate electrode and the conductive film 29 b functioningas a gate electrode have the same potential.

Thus, by applying voltage at the same potential to each gate electrodeof the transistor 102 a, variation in the initial characteristics can bereduced, and degradation of the transistor 102 a after the −GBT stresstest and a change in the rising voltage of on-state current at differentdrain voltages can be suppressed. In addition, a region where carriersflow in the oxide semiconductor film 19 a is increased in the filmthickness direction, so that the amount of transferred carriers isincreased. As a result, the on-state current of the transistor 102 a isincreased, and the field-effect mobility is increased. Typically, thefield-effect mobility is greater than or equal to 20 cm²/Vs.

Over the transistor 102 a in this embodiment, the oxide insulating films23 and 25, which are subjected to element isolation, are formed. Theoxide insulating films 23 and 25 overlap with the oxide semiconductorfilm 19 a. In the cross-sectional view in the channel width direction,end portions of the oxide insulating films 23 and 25 are located on anouter side of the oxide semiconductor film 19 a. Furthermore, in thechannel width direction in FIG. 12 , the conductive film 29 bfunctioning as a gate electrode faces a side surface of the oxidesemiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween.

An end portion processed by etching or the like of the oxidesemiconductor film, in which defects are generated by damage due toprocessing, is also contaminated by the attachment of an impurity, orthe like. Thus, the end portion of the oxide semiconductor film iseasily activated by application of a stress such as an electric field,thereby easily becoming n-type (having a low resistance). Therefore, theend portion of the oxide semiconductor film 19 a overlapping with theconductive film 13 functioning as a gate electrode easily becomesn-type. When the end portion which becomes n-type is provided betweenthe conductive films 21 a and 21 b functioning as a source electrode anda drain electrode, the region which becomes n-type serves as a carrierpath, resulting in a parasitic channel. However, as illustrated in thecross-sectional view along line C-D, when the conductive film 29 bfunctioning as a gate electrode faces a side surface of the oxidesemiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween in the channel width direction, due to theelectric field of the conductive film 29 b functioning as a gateelectrode, generation of a parasitic channel on the side surface of theoxide semiconductor film 19 a or in a region including the side surfaceand the vicinity of the side surface is suppressed. As a result, atransistor which has excellent electrical characteristics such as asharp increase in the drain current at the threshold voltage isobtained.

The common electrode includes the stripe regions extending in adirection intersecting with a signal line. Accordingly, in the vicinityof the pixel electrode 19 b and the conductive film 21 a, unintendedalignment of liquid crystal molecules can be prevented and thus lightleakage can be suppressed. As a result, a display device with excellentcontrast can be manufactured.

In the capacitor 105 a, the pixel electrode 19 b is formed at the sametime as the oxide semiconductor film 19 a and has increased conductivityby containing an impurity. Alternatively, the pixel electrode 19 b isformed at the same time as the oxide semiconductor film 19 a, and hasincreased conductivity by containing oxygen vacancies generated byplasma damage or the like. Alternatively, the pixel electrode 19 b isformed at the same time as the oxide semiconductor film 19 a, and hasincreased conductivity by containing impurities and oxygen vacanciesgenerated by plasma damage or the like.

On the element substrate of the display device in this embodiment, thepixel electrode is formed at the same time as the oxide semiconductorfilm of the transistor. The pixel electrode functions as one ofelectrodes of the capacitor. The common electrode functions as the otherof electrodes of the capacitor. Thus, a step of forming anotherconductive film is not needed to form the capacitor, and the number ofsteps of manufacturing the display device can be reduced. The capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

Details of the transistor 102 a are described below. Note that thedescription of the components with the same reference numerals as thosein Embodiment 2 is omitted.

The conductive film 29 b functioning as a gate electrode can be formedusing a material similar to that of the common electrode 29 inEmbodiment 2.

Next, a method for manufacturing the transistor 102 a and the capacitor105 a in FIG. 12 is described with reference to FIGS. 6A to 6D, FIGS. 7Ato 7D, FIG. 8A, and FIGS. 13A to 13C.

As in Embodiment 2, through the steps illustrated in FIGS. 6A to 6D,FIGS. 7A to 7D, and FIG. 8A, the conductive film 13 functioning as agate electrode, the nitride insulating film 15, the oxide insulatingfilm 16, the oxide semiconductor film 19 a, the pixel electrode 19 b,the conductive films 21 a and 21 b functioning as a source electrode anda drain electrode, the oxide insulating film 22, the oxide insulatingfilm 24, and the nitride insulating film 26 are formed over thesubstrate 11. In these steps, the photography processes using the firstto fourth photomasks are performed.

Next, a mask is formed over the nitride insulating film 26 through aphotolithography process using a fifth photomask, and then part of thenitride insulating film 26 is etched using the mask; thus, the nitrideinsulating film 27 having the opening portions 41 a and 41 b is formedas illustrated in FIG. 13A.

Next, as illustrated in FIG. 13B, the conductive film 28 to be thecommon electrode 29 and the conductive film 29 b functioning as a gateelectrode is formed over the conductive film 13 functioning as a gateelectrode, the conductive film 21 b, and the nitride insulating film 27.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 13C, partof the conductive film 28 is etched with the use of the mask to form thecommon electrode 29 and the conductive film 29 b functioning as a gateelectrode. After that, the mask is removed.

Through the above process, the transistor 102 a is manufactured and thecapacitor 105 a can also be manufactured.

In the transistor described in this embodiment, since the commonelectrode 29 functioning as a gate electrode faces a side surface of theoxide semiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween in the channel width direction, due to theelectric field of the conductive film 29 b functioning as a gateelectrode, generation of a parasitic channel on the side surface of theoxide semiconductor film 19 a or in a region including the side surfaceand the vicinity of the side surface is suppressed. As a result, atransistor which has excellent electrical characteristics such as asharp increase in the drain current at the threshold voltage isobtained.

The element substrate of the display device of this embodiment isprovided with a common electrode including stripe regions extending in adirection intersecting with a signal line. Therefore, the display devicecan have excellent contrast.

On the element substrate of the display device in this embodiment, thepixel electrode is formed at the same time as the oxide semiconductorfilm of the transistor. The pixel electrode functions as one ofelectrodes of the capacitor. The common electrode functions as the otherof electrodes of the capacitor. Thus, a step of forming anotherconductive film is not needed to form the capacitor, and the number ofsteps of manufacturing the display device can be reduced. The capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 6

As for the conductive films 21 a and 21 b functioning as a sourceelectrode and a drain electrode provided in each of the transistorsdescribed in Embodiments 2 to 5, it is possible to use a conductivematerial which is easily bonded to oxygen, such as tungsten, titanium,aluminum, copper, molybdenum, chromium, or tantalum, or an alloythereof. Thus, oxygen contained in the oxide semiconductor film 19 a andthe conductive material contained in the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode are bonded toeach other, so that an oxygen vacancy region is formed in the oxidesemiconductor film 19 a. Further, in some cases, part of constituentelements of the conductive material that forms the conductive films 21 aand 21 b functioning as a source electrode and a drain electrode ismixed into the oxide semiconductor film 19 a. Consequently,low-resistance regions are formed in the vicinity of regions of theoxide semiconductor film 19 a which are in contact with the conductivefilms 21 a and 21 b functioning as a source electrode and a drainelectrode. The low-resistance regions are formed between the oxideinsulating film 17 and the conductive films 21 a and 21 b functioning asa source electrode and a drain electrode so as to be in contact with theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode. Since the low-resistance regions have highconductivity, contact resistance between the oxide semiconductor film 19a and the conductive films 21 a and 21 b functioning as a sourceelectrode and a drain electrode can be reduced, and thus, the on-statecurrent of the transistor can be increased.

Further, the conductive films 21 a and 21 b functioning as a sourceelectrode and a drain electrode may each have a stacked-layer structureof the conductive material which is easily bonded to oxygen and aconductive material which is not easily bonded to oxygen, such astitanium nitride, tantalum nitride, or ruthenium. With such astacked-layer structure, oxidization of the conductive films 21 a and 21b functioning as a source electrode and a drain electrode can beprevented at the interface between the oxide insulating film 23 and theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, so that an increase in the resistance of the conductivefilms 21 a and 21 b functioning as a source electrode and a drainelectrode can be inhibited.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 7

In this embodiment, a display device including a transistor in which theamount of defects in an oxide semiconductor film can be further reducedas compared to Embodiments 2 to 5 is described with reference todrawings. The transistor described in this embodiment is different fromany of the transistors in Embodiments 2 to 5 in that a multilayer filmincluding a plurality of oxide semiconductor films is provided. Here,details are described using the transistor in Embodiment 2.

FIGS. 14A and 14B each illustrate a cross-sectional view of an elementsubstrate included in a display device. FIGS. 14A and 14B arecross-sectional views taken along lines A-B and C-D in FIG. 4 .

A transistor 102 b in FIG. 14A includes a multilayer film 37 aoverlapping with the conductive film 13 functioning as a gate electrodewith the nitride insulating film 15 and the oxide insulating film 17provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode in contact withthe multilayer film 37 a. The oxide insulating film 23, the oxideinsulating film 25, and the nitride insulating film 27 are formed overthe nitride insulating film 15, the oxide insulating film 17, themultilayer film 37 a, and the conductive films 21 a and 21 b functioningas a source electrode and a drain electrode.

The capacitor 105 b in FIG. 14A includes a multilayer film 37 b formedover the oxide insulating film 17, the nitride insulating film 27 incontact with the multilayer film 37 b, and the common electrode 29 incontact with the nitride insulating film 27. The multilayer film 37 bincludes an oxide semiconductor film 19 f and an oxide semiconductorfilm 39 b. In other words, the multilayer film 37 b has a two-layerstructure. The multilayer film 37 b functions as a pixel electrode.

In the transistor 102 b described in this embodiment, the multilayerfilm 37 a includes the oxide semiconductor film 19 a and an oxidesemiconductor film 39 a. That is, the multilayer film 37 a has atwo-layer structure. Part of the oxide semiconductor film 19 a serves asa channel region. Furthermore, the oxide insulating film 23 is formed incontact with the oxide semiconductor film 39 a, and the oxide insulatingfilm 25 is formed in contact with the oxide insulating film 23. That is,the oxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23.

The oxide semiconductor film 39 a is an oxide film containing one ormore elements that form the oxide semiconductor film 19 a. Thus,interface scattering is unlikely to occur at the interface between theoxide semiconductor films 19 a and 39 a. Accordingly, the transistor canhave high field-effect mobility because the movement of carriers is nothindered at the interface.

The oxide semiconductor film 39 a is typically an In—Ga oxide film, anIn—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ga, Y, Zr,La, Ce, or Nd). The energy at the conduction band bottom of the oxidesemiconductor film 39 a is closer to a vacuum level than that of theoxide semiconductor film 19 a is, and typically, the difference betweenthe energy at the conduction band bottom of the oxide semiconductor film39 a and the energy at the conduction band bottom of the oxidesemiconductor film 19 a is any one of 0.05 eV or more, 0.07 eV or more,0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV orless, 0.5 eV or less, or 0.4 eV or less. That is, the difference betweenthe electron affinity of the oxide semiconductor film 39 a and theelectron affinity of the oxide semiconductor film 19 a is any one of0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more,and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV orless.

The oxide semiconductor film 39 a preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 39 a contains a larger amount of Al,Ga, Y, Zr, La, Ce, or Nd in an atomic ratio than the amount of In in anatomic ratio, any of the following effects may be obtained: (1) theenergy gap of the oxide semiconductor film 39 a is widened; (2) theelectron affinity of the oxide semiconductor film 39 a is reduced; (3)scattering of impurities from the outside is reduced; (4) an insulatingproperty increases as compared to the oxide semiconductor films 19 a;and (5) oxygen vacancies are less likely to be generated because Al, Ga,Y, Zr, La, Ce, and Nd are metal elements which are strongly bonded tooxygen.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxidefilm, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are preferably as follows: the atomic percentage ofIn is less than 50 atomic % and the atomic percentage of M is greaterthan or equal to 50 atomic %, or more preferably, the atomic percentageof In is less than 25 atomic % and the atomic percentage of M is greaterthan or equal to 75 atomic %.

Furthermore, in the case where each of the oxide semiconductor films 19a and 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Cc,or Nd), the proportion of M atoms (M represents Al, Ga, Y, Zr, La, Ce,or Nd) in the oxide semiconductor film 39 a is higher than that in theoxide semiconductor film 19 a. As a typical example, the proportion of Min the oxide semiconductor film 39 a is 1.5 or more times, preferablytwice or more, further preferably three or more times as high as that inthe oxide semiconductor film 19 a.

Furthermore, in the case where each of the oxide semiconductor films 19a and 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce,or Nd), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] is satisfied in the oxidesemiconductor film 39 a and In:M:Zn=x₂:y₂:z₂ [atomic ratio] is satisfiedin the oxide semiconductor film 19 a, y₁/x₁ is higher than y₂/x₂, andy₁/x₁ is preferably 1.5 or more times, more preferably twice or more,still more preferably three or more time as high as y₂/x₂.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for formingthe oxide semiconductor film 19 a, x₁/y₁ is preferably greater than orequal to 1/3 and less than or equal to 6, further preferably greaterthan or equal to 1 and less than or equal to 6, and z₁/y₁ is preferablygreater than or equal to 1/3 and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a CAAC-OS film as the oxide semiconductor film 19 a is easily formed.Typical examples of the atomic ratio of the metal elements of the targetare In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for formingthe oxide semiconductor film 39 a, x₂/y₂ is preferably less than x₁/y₁,and z₂/y₂ is preferably greater than or equal to 1/3 and less than orequal to 6, further preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₂/y₂ is greater than or equal to 1 andless than or equal to 6, a CAAC-OS film as the oxide semiconductor film39 a is easily formed. Typical examples of the atomic ratio of the metalelements of the target are In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6,In:M:Zn=1:3:8, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:6:8.

Note that the proportion of each metal element in the atomic ratio ofeach of the oxide semiconductor films 19 a and the oxide semiconductorfilm 39 a varies within a range of ±40% of that in the above atomicratio as an error.

The oxide semiconductor film 39 a also functions as a film that relievesdamage to the oxide semiconductor film 19 a at the time of forming theoxide insulating film 25 later.

The thickness of the oxide semiconductor film 39 a is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 39 a may have a non-single-crystalstructure, for example, like the oxide semiconductor film 19 a. Thenon-single-crystal structure includes a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) which is described later, a polycrystallinestructure, a microcrystalline structure which is described later, or anamorphous structure, for example.

The oxide semiconductor film 39 a may have an amorphous structure, forexample. The oxide semiconductor films having the amorphous structureeach have disordered atomic arrangement and no crystalline component,for example.

Note that the oxide semiconductor films 19 a and 39 a may each be amixed film including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Further, in some cases, the mixed film has a stacked-layer structure inwhich two or more of the following regions are stacked: a region havingan amorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure.

Here, the oxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23. Thus, ifcarrier traps are formed between the oxide semiconductor film 39 a andthe oxide insulating film 23 by impurities and defects, electronsflowing in the oxide semiconductor film 19 a are less likely to becaptured by the carrier traps because there is a distance between thecarrier traps and the oxide semiconductor film 19 a. Accordingly, theamount of on-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are capturedby the carrier traps, the electrons become negative fixed charges. As aresult, a threshold voltage of the transistor fluctuates. However, bythe distance between the oxide semiconductor film 19 a and the carriertraps, capture of electrons by the carrier traps can be reduced, andaccordingly, fluctuations of the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Furthermore, an oxygen vacancy is less likely to be formed inthe oxide semiconductor film 39 a. Consequently, the impurityconcentration and the amount of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced.

Note that the oxide semiconductor films 19 a and 39 a are not formed bysimply stacking each film, but are formed to form a continuous junction(here, in particular, a structure in which the energy of the conductionband bottom is changed continuously between each film). In other words,a stacked-layer structure in which there exists no impurity that forms adefect state such as a trap center or a recombination center at eachinterface is provided. If an impurity exists between the oxidesemiconductor films 19 a and 39 a that are stacked, a continuity of theenergy band is damaged, and the carrier is captured or recombined at theinterface and then disappears.

In order to form such a continuous junction, it is necessary to formfilms continuously without being exposed to air, with the use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

As in a transistor 102 c in FIG. 14B, a multilayer film 38 a may beprovided instead of the multilayer film 37 a.

In addition, as in a capacitor 105 c in FIG. 14B, a multilayer film 38 bmay be provided instead of the multilayer film 37 b.

The multilayer film 38 a includes an oxide semiconductor film 49 a, theoxide semiconductor film 19 a, and the oxide semiconductor film 39 a.That is, the multilayer film 38 a has a three-layer structure. Further,the oxide semiconductor film 19 a serves as a channel region.

The oxide semiconductor film 49 a can be formed using a material and aformation method similar to those of the oxide semiconductor film 39 aas appropriate.

The multilayer film 38 b includes an oxide semiconductor film 49 b, theoxide semiconductor film 19 f, and the oxide semiconductor film 39 b. Inother words, the multilayer film 38 b has a three-layer structure. Themultilayer film 38 b functions as a pixel electrode.

The oxide semiconductor film 19 f can be formed using a material and aformation method similar to those of the pixel electrode 19 b asappropriate. The oxide semiconductor film 49 b can be formed using amaterial and a formation method similar to those of the oxidesemiconductor film 39 b as appropriate.

Furthermore, the oxide insulating film 17 and the oxide semiconductorfilm 49 a are in contact with each other. That is, the oxidesemiconductor film 49 a is provided between the oxide insulating film 17and the oxide semiconductor film 19 a.

The multilayer film 38 a and the oxide insulating film 23 are in contactwith each other. In addition, the oxide semiconductor film 39 a and theoxide insulating film 23 are in contact with each other. That is, theoxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23.

It is preferable that the thickness of the oxide semiconductor film 49 abe smaller than that of the oxide semiconductor film 19 a. When thethickness of the oxide semiconductor film 49 a is greater than or equalto 1 nm and less than or equal to 5 nm, preferably greater than or equalto 1 nm and less than or equal to 3 nm, the amount of change in thethreshold voltage of the transistor can be reduced.

In the transistor described in this embodiment, the oxide semiconductorfilm 39 a is provided between the oxide semiconductor film 19 a and theoxide insulating film 23. Thus, if carrier traps are formed between theoxide semiconductor film 39 a and the oxide insulating film 23 byimpurities and defects, electrons flowing in the oxide semiconductorfilm 19 a are less likely to be captured by the carrier traps becausethere is a distance between the carrier traps and the oxidesemiconductor film 19 a. Accordingly, the amount of on-state current ofthe transistor can be increased, and the field-effect mobility can beincreased. When the electrons are captured by the carrier traps, theelectrons become negative fixed charges. As a result, a thresholdvoltage of the transistor changes. However, by the distance between theoxide semiconductor film 19 a and the carrier traps, capture ofelectrons by the carrier traps can be reduced, and accordingly, changeof the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Further, an oxygen vacancy is less likely to be formed in theoxide semiconductor film 39 a. Consequently, the impurity concentrationand the amount of oxygen vacancies in the oxide semiconductor film 19 acan be reduced.

Further, the oxide film 49 a is provided between the oxide insulatingfilm 17 and the oxide semiconductor film 19 a, and the oxidesemiconductor film 39 a is provided between the oxide semiconductor film19 a and the oxide insulating film 23. Thus, it is possible to reducethe concentration of silicon or carbon in the vicinity of the interfacebetween the oxide semiconductor film 49 a and the oxide semiconductorfilm 19 a, the concentration of silicon or carbon in the oxidesemiconductor film 19 a, or the concentration of silicon or carbon inthe vicinity of the interface between the oxide semiconductor film 39 aand the oxide semiconductor film 19 a. Consequently, in the multilayerfilm 38 a, the absorption coefficient derived from a constantphotocurrent method is lower than 1×10⁻³/cm, preferably lower than1×10⁻⁴/cm, and thus density of localized levels is extremely low.

The transistor 102 c having such a structure includes very few defectsin the multilayer film 38 a; thus, the electrical characteristics of thetransistor can be improved, and typically, the on-state current can beincreased and the field-effect mobility can be improved. Furthermore, ina BT stress test and a BT photostress test which are examples of astress test, the amount of change in threshold voltage is small, andthus, reliability is high.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 8

In this embodiment, one embodiment that can be applied to an oxidesemiconductor film in the transistor included in the display devicedescribed in the above embodiment is described.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor); and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Further, the oxide semiconductor film may include aCAAC-OS. Furthermore, the oxide semiconductor film may include anamorphous oxide semiconductor and an oxide semiconductor having acrystal grain. Described below are a CAAC-OS and a microcrystallineoxide semiconductor as typical examples.

<CAAC-OS>

The CAAC-OS film is one of oxide semiconductor films having a pluralityof crystal parts. The crystal parts included in the CAAC-OS film eachhave c-axis alignment. In a plan TEM image, the area of the crystalparts included in the CAAC-OS film is greater than or equal to 2500 nm²,preferably greater than or equal to 5 μm², further preferably greaterthan or equal to 1000 μm². Furthermore, in a cross-sectional TEM image,when the proportion of the crystal parts is greater than or equal to50%, preferably greater than or equal to 80%, further preferably greaterthan or equal to 95% of the CAAC-OS film, the CAAC-OS film is a thinfilm having physical properties similar to those of a single crystal.

In a transmission electron microscope (TEM) image of the CAAC-OS film,it is difficult to clearly observe a boundary between crystal parts,that is, a grain boundary. Thus, in the CAAC-OS film, a reduction inelectron mobility due to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflecting unevenness of a surfaceover which the CAAC-OS film is formed (hereinafter, a surface over whichthe CAAC-OS film is formed is referred to as a formation surface) or atop surface of the CAAC-OS film, and is arranged parallel to theformation surface or the top surface of the CAAC-OS film. In thisspecification, a term “parallel” indicates that the angle formed betweentwo straight lines is greater than or equal to −10° and less than orequal to 10°, and accordingly also includes the case where the angle isgreater than or equal to −5° and less than or equal to 5°. In addition,a term “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(luminescent spots) having alignment are shown.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. When the CAAC-OS film is analyzed by anout-of-plane method, a peak appears frequently when the diffractionangle (2θ) is around 31°. This peak is derived from the (00x) plane (xis an integral number) of the In—Ga—Zn oxide crystal, which indicatesthat crystals in the CAAC-OS film have c-axis alignment, and that thec-axes are aligned in a direction substantially perpendicular to theformation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the In—Ga—Zn oxidecrystal. Here, analysis (ϕ scan) is performed under conditions where thesample is rotated around a normal vector of a sample surface as an axis(0 axis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of In—Ga—Zn oxide, six peaksappear. The six peaks are derived from crystal planes equivalent to the(110) plane. On the other hand, in the case of a CAAC-OS film, a peak isnot clearly observed even when 0 scan is performed with 2θ fixed ataround 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of the formation surface or anormal vector of the top surface of the CAAC-OS film. Thus, for example,in the case where a shape of the CAAC-OS film is changed by etching orthe like, the c-axis might not be necessarily parallel to a normalvector of a formation surface or a normal vector of a top surface of theCAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film is analyzed by an out-of-plane method, apeak of 2θ may also be observed at around 36°, in addition to the peakof 2θ at around 31°. The peak of 2θ at around 36° indicates that acrystal part having no c-axis alignment is included in part of theCAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2σappear at around 31° and a peak of 2θ not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (or molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the amount of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<Microcrystalline Oxide Semiconductor>

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor film in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor filmis greater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. An oxidesemiconductor film including nanocrystal (nc), which is a microcrystalwith a size greater than or equal to 1 nm and less than or equal to 10nm, or a size greater than or equal to 1 nm and less than or equal to 3nm, is specifically referred to as a nanocrystalline oxide semiconductor(nc-OS) film. In an image obtained with TEM, a crystal grain boundarycannot be found clearly in the nc-OS film in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter larger than the diameterof a crystal part (e.g., larger than or equal to 50 nm). Meanwhile,spots are shown in a nanobeam electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter (e.g.,larger than or equal to 1 nm and smaller than or equal to 30 nm) closeto or smaller than the diameter of a crystal part. Further, in ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are observed in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots is shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

<Oxide Semiconductor Film and Oxide Conductor Film>

Next, the temperature dependence of conductivity of a film formed withan oxide semiconductor (hereinafter referred to as an oxidesemiconductor film (OS)) and that of a film formed with an oxideconductor (hereinafter referred to as an oxide conductor film (OC)),which can be used for the pixel electrode 19 b, will be described withreference to FIG. 26 . In FIG. 26 , the horizontal axes representmeasurement temperature (the lower horizontal axis represents 1/T andthe upper horizontal axis represents T), and the vertical axisrepresents conductivity (1/p). Measurement results of the oxidesemiconductor film (OS) are plotted as triangles, and measurementresults of the oxide conductor film (OC) are plotted as circles.

Note that a sample including the oxide semiconductor film (OS) wasprepared by forming a 35-nm-thick In—Ga—Zn oxide film over a glasssubstrate by a sputtering method using a sputtering target with anatomic ratio of In:Ga:Zn=1:1:1.2, forming a 20-nm-thick In—Ga—Zn oxidefilm over the 35-nm-thick In—Ga—Zn oxide film by a sputtering methodusing a sputtering target with an atomic ratio of In:Ga:Zn=1:4:5,performing heat treatment in a 450° C. nitrogen atmosphere and thenperforming heat treatment in a 450° C. atmosphere of a mixed gas ofnitrogen and oxygen, and forming a silicon oxynitride film over theoxide films by a plasma CVD method.

A sample including the oxide conductor film (OC) was prepared by forminga 100-nm-thick In—Ga—Zn oxide film over a glass substrate by asputtering method using a sputtering target with an atomic ratio ofIn:Ga:Zn=1:1:1, performing heat treatment in a 450° C. nitrogenatmosphere and then performing heat treatment in a 450° C. atmosphere ofa mixed gas of nitrogen and oxygen, and forming a silicon nitride filmover the oxide film by a plasma CVD method.

As can be seen from FIG. 26 , the temperature dependence of conductivityof the oxide conductor film (OC) is lower than the temperaturedependence of conductivity of the oxide semiconductor film (OS).Typically, the range of variation of conductivity of the oxide conductorfilm (OC) at temperatures from 80 K to 290 K is from more than ˜20% toless than +20%. Alternatively, the range of variation of conductivity attemperatures from 150 K to 250 K is from more than ˜10% to less than+10%. In other words, the oxide conductor is a degenerate semiconductorand it is suggested that the conduction band minimum agrees with orsubstantially agrees with the Fermi level. Therefore, the oxideconductor film (OC) can be used for a resistor, a wiring, an electrode,a pixel electrode, a common electrode, or the like.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 9

In the method for manufacturing any of the transistors described in theabove embodiments, after the conductive films 21 a and 21 b functioningas a source electrode and a drain electrode are formed, the oxidesemiconductor film 19 a may be exposed to plasma generated in anoxidizing atmosphere, so that oxygen may be supplied to the oxidesemiconductor film 19 a. Atmospheres of oxygen, ozone, dinitrogenmonoxide, nitrogen dioxide, and the like can be given as examples ofoxidizing atmospheres. Further, in the plasma treatment, the oxidesemiconductor film 19 a is preferably exposed to plasma generated withno bias applied to the substrate 11 side. Consequently, the oxidesemiconductor film 19 a can be supplied with oxygen without beingdamaged; accordingly, the amount of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced. Moreover, impurities, e.g.,halogen such as fluorine or chlorine, remaining on a surface of theoxide semiconductor film 19 a due to the etching treatment can beremoved. The plasma treatment is preferably performed while heating isperformed at a temperature higher than or equal to 300° C. Oxygen in theplasma is bonded to hydrogen contained in the oxide semiconductor film19 a to form water. Since the substrate is heated, the water is releasedfrom the oxide semiconductor film 19 a. Consequently, the amount ofhydrogen and water in the oxide semiconductor film 19 a can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 10

In this embodiment, structural examples of electronic appliances eachusing a display device of one embodiment of the present invention willbe described. In addition, in this embodiment, a display module using adisplay device of one embodiment of the present invention will bedescribed with reference to FIG. 15 .

In a display module 8000 in FIG. 15 , a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002. Note thatthe backlight unit 8007, the battery 8011, the touch panel 8004, and thelike are not provided in some cases.

The display device of one embodiment of the present invention can beused for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed so as to overlap with the display panel8006. A counter substrate (sealing substrate) of the display panel 8006can have a touch panel function. A photosensor may be provided in eachpixel of the display panel 8006 to form an optical touch panel. Anelectrode for a touch sensor may be provided in each pixel of thedisplay panel 8006 so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 16A to 16D are each an external view of an electronic applianceincluding a display device of one embodiment of the present invention.

Examples of electronic appliances are a television set (also referred toas a television or a television receiver), a monitor of a computer orthe like, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone (also referred to as a cellularphone or a cellular phone device), a portable game machine, a portableinformation terminal, an audio reproducing device, a large-sized gamemachine such as a pachinko machine, and the like.

FIG. 16A illustrates a portable information terminal including a mainbody 1001, a housing 1002, display portions 1003 a and 1003 b, and thelike. The display portion 1003 b is a touch panel. By touching akeyboard button 1004 displayed on the display portion 1003 b, a screencan be operated, and text can be input. It is needless to say that thedisplay portion 1003 a may be a touch panel. A liquid crystal panel oran organic light-emitting panel is manufactured by using any of thetransistors described in the above embodiments as a switching elementand used in the display portion 1003 a or 1003 b, whereby a highlyreliable portable information terminal can be provided.

The portable information terminal illustrated in FIG. 16A can have afunction of displaying a variety of kinds of data (e.g., a still image,a moving image, and a text image), a function of displaying a calendar,a date, the time, or the like on the display portion, a function ofoperating or editing data displayed on the display portion, a functionof controlling processing by a variety of kinds of software (programs),and the like. Furthermore, an external connection terminal (an earphoneterminal, a USB terminal, or the like), a recording medium insertionportion, and the like may be provided on the back surface or the sidesurface of the housing.

The portable information terminal illustrated in FIG. 16A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an e-bookserver.

FIG. 16B illustrates a portable music player, which includes in a mainbody 1021, a display portion 1023, a fixing portion 1022 with which theportable music player can be worn on the ear, a speaker, an operationbutton 1024, an external memory slot 1025, and the like. A liquidcrystal panel or an organic light-emitting panel is manufactured usingany of the transistors described in the above embodiments as a switchingelement, and used in the display portion 1023, whereby a highly reliableportable music player can be provided.

Furthermore, when the portable music player illustrated in FIG. 16B hasan antenna, a microphone, or a wireless communication function and isused with a mobile phone, a user can talk wirelessly and hands-freely onthe phone while driving a car or the like.

FIG. 16C illustrates a mobile phone, which includes two housings, ahousing 1030 and a housing 1031. The housing 1031 includes a displaypanel 1032, a speaker 1033, a microphone 1034, a pointing device 1036, acamera lens 1037, an external connection terminal 1038, and the like.The housing 1030 is provided with a solar cell 1040 for charging themobile phone, an external memory slot 1041, and the like. In addition,an antenna is incorporated in the housing 1031. Any of the transistorsdescribed in the above embodiments is used in the display panel 1032,whereby a highly reliable mobile phone can be provided.

Further, the display panel 1032 includes a touch panel. A plurality ofoperation keys 1035 displayed as images is indicated by dashed lines inFIG. 16C. Note that a boosting circuit by which voltage output from thesolar cell 1040 is increased to be sufficiently high for each circuit isalso included.

In the display panel 1032, the direction of display is changed asappropriate depending on the application mode. Further, the mobile phoneis provided with the camera lens 1037 on the same surface as the displaypanel 1032, and thus it can be used as a video phone. The speaker 1033and the microphone 1034 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousings 1030 and 1031 in a state where they are developed asillustrated in FIG. 16C can shift, by sliding, to a state where one islapped over the other. Therefore, the size of the mobile phone can bereduced, which makes the mobile phone suitable for being carried around.

The external connection terminal 1038 can be connected to an AC adaptorand a variety of cables such as a USB cable, whereby charging and datacommunication with a personal computer or the like are possible.Further, by inserting a recording medium into the external memory slot1041, a larger amount of data can be stored and moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 16D illustrates an example of a television set. In a television set1050, a display portion 1053 is incorporated in a housing 1051. Imagescan be displayed on the display portion 1053. Moreover, a CPU isincorporated in a stand 1055 for supporting the housing 1051. Any of thetransistors described in the above embodiments is used in the displayportion 1053 and the CPU, whereby the television set 1050 can have highreliability.

The television set 1050 can be operated with an operation switch of thehousing 1051 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 1050 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

Further, the television set 1050 is provided with an external connectionterminal 1054, a storage medium recording and reproducing portion 1052,and an external memory slot. The external connection terminal 1054 canbe connected to various types of cables such as a USB cable, and datacommunication with a personal computer or the like is possible. A diskstorage medium is inserted into the storage medium recording andreproducing portion 1052, and reading data stored in the storage mediumand writing data to the storage medium can be performed. In addition, animage, a video, or the like stored as data in an external memory 1056inserted into the external memory slot can be displayed on the displayportion 1053.

Further, in the case where the off-state leakage current of thetransistor described in the above embodiments is extremely small, whenthe transistor is used in the external memory 1056 or the CPU, thetelevision set 1050 can have high reliability and sufficiently reducedpower consumption.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Example 1

In this example, distribution of transmittance of a pixel included in aliquid crystal display device according to one embodiment of the presentinvention was evaluated by calculation.

First, samples used in this example are described.

FIG. 17A is a top view of Sample 1 which is a comparative example. Thearea of a pixel in Sample 1 includes a scan line 201 and a common line203 which extend in a horizontal direction, a signal line 205 whichextends in a vertical direction (a direction perpendicular to the scanline and the common line), and a region surrounded by these lines. Onepixel is 84 μm long and 28 μm wide.

Sample 1 includes a common electrode 207 which is placed inside a regionsurrounded by the above-mentioned lines and a signal line of ahorizontally adjacent pixel and electrically connected to the commonline 203, and a comb-like pixel electrode 209 placed over the commonelectrode 207. Teeth of the pixel electrode 209 extend in a directionintersecting with the signal line 205. In Sample 1, the pixel isprovided with a transistor including a gate electrode electricallyconnected to the scan line 201, a semiconductor film 211 which overlapswith the gate electrode with a gate insulating film providedtherebetween and is formed through the same process as the commonelectrode 207, a source electrode electrically connected to thesemiconductor film 211 and the signal line 205, and a drain electrode213 electrically connected to the semiconductor film 211 and the pixelelectrode 209.

FIG. 17B is a top view of Sample 2 which is one embodiment of thepresent invention. The area of a pixel in Sample 2 includes a scan line221 extending in a horizontal direction, a signal line 225 extending ina vertical direction, and a region surrounded by these lines. One pixelis 84 μm long and 28 μm wide.

Sample 2 includes a pixel electrode 229 which is placed inside a regionsurrounded by the above-mentioned lines, a signal line of a horizontallyadjacent pixel, and a scan line of a vertically adjacent pixel, and acommon electrode 227 placed over the pixel electrode 229. The commonelectrode 227 includes stripe regions extending in a directionintersecting with the signal line 225. In Sample 2, the pixel isprovided with a transistor including a gate electrode electricallyconnected to the scan line 221, a semiconductor film 231 which overlapswith the gate electrode with a gate insulating film providedtherebetween and is formed through the same process as the pixelelectrode 229, a source electrode electrically connected to thesemiconductor film 231 and the signal line 225, and a drain electrode233 electrically connected to the semiconductor film 231 and the pixelelectrode 229. The transistor 102 described in Embodiment 2 andillustrated in FIG. 5 can be referred to for the cross-sectional shapeof the transistor.

Sample 1 and Sample 2 were prepared in the above manner. Transmittanceof liquid crystals in the pixels of Sample 1 and Sample 2 can becontrolled by a horizontal electric field applied between the pixelelectrode and the common electrode.

Next, the transmittance of Sample 1 and Sample 2 were calculated. Thecalculation was performed using LCDMaster 3D (produced by SHINTECH,Inc.) in an FEM-Static mode. In the calculation, the size was 84 μmlong, 28 μm wide, and 4 μm deep (high), and the boundary condition was aperiodic boundary condition. The gate electrode was 200 nm thick, thegate insulating film was 400 nm thick, the signal line was 300 nm thick,and an interlayer insulating film was 500 nm thick in each of Sample 1and Sample 2. In Sample 1, the common electrode was 0 nm thick, anitride insulating film between the common electrode and the pixelelectrode was 100 nm thick, and the pixel electrode was 100 nm thick. InSample 2, the pixel electrode was 0 nm thick, a nitride insulating filmbetween the pixel electrode and the common electrode was 100 nm thick,and the common electrode was 100 nm thick. The rubbing direction of theliquid crystal was 85°, the twist angle was 0°, and the pretilt anglewas 3°. Note that the common electrode of Sample 1 and the pixelelectrode of Sample 2 were each 0 nm thick in order to reducecalculation load. The distribution of transmittance in the case where −9V was applied to the scan line, 0 V was applied to the common line, and6 V was applied to the signal line and the pixel electrode under theabove conditions was evaluated.

The distribution of transmittance is expressed by grayscalc; a whiterregion has higher transmittance. FIG. 17C shows the distribution oftransmittance of Sample 1 and FIG. 17D shows the distribution oftransmittance of Sample 2.

It is found that regions with high transmittance were formed in Sample 1and Sample 2. In particular, a region with high transmittance was formedin a wide area in the pixel in Sample 2. This is because the commonelectrode of Sample 2 does not include a region extending in a directionparallel to the signal line and an electric field between the pixelelectrode and the common electrode is generated in a wider region inSample 2 than in Sample 1.

Thus, Sample 2 is an effective structure for a liquid crystal displaydevice with low power consumption.

Example 2

In this example, light leakage in a black display region when white andblack are displayed in adjacent pixels in a liquid crystal displaydevice according to one embodiment of the present invention wasevaluated by calculation.

First, samples used in this example are described.

FIG. 18A is a top view of Sample 3. The area of a pixel of Sample 3includes a scan line 241 extending in a horizontal direction, a signalline 243 extending in a vertical direction, and a region surrounded bythese lines. The area of two horizontally adjacent pixels is 49.5 μmlong and 30 μm wide.

Sample 3 includes a pixel electrode 249 placed inside a regionsurrounded by the above lines, a signal line of a horizontally adjacentpixel, and a scan line of a vertically adjacent pixel, and a commonelectrode 247 placed over the pixel electrode 249. The common electrode247 includes stripe regions extending in a direction intersecting withthe signal line 243. In Sample 3, the pixel is provided with atransistor including a gate electrode electrically connected to the scanline 241, a semiconductor film 251 which overlaps with the gateelectrode with a gate insulating film provided therebetween and isformed through the same process as the pixel electrode 249, a sourceelectrode electrically connected to the semiconductor film 251 and thesignal line 243, and a drain electrode 253 electrically connected to thesemiconductor film 251 and the pixel electrode. The transistor 102described in Embodiment 2 and illustrated in FIG. 5 can be referred tofor the cross-sectional shape of the transistor.

FIG. 18B is a top view of Sample 4. Sample 4 has a structure similar tothe structure of Sample 3 except for the shapes of the drain electrodeand the common electrode. Specifically, in Sample 4, a drain electrode263 has an L shape and overlaps with an end portion of the pixelelectrode 249. Thus, the influence of an electric field generatedbetween the scan line 241 and the pixel electrode 249 is reduced.Furthermore, a common electrode 267 is connected to a verticallyadjacent pixel across the scan line 241, whereby the influence of anelectric field generated between the scan line 241 and the pixelelectrode 249 is reduced.

Sample 3 and Sample 4 were prepared in the above manner. Transmittanceof liquid crystal elements in the pixels of Sample 3 and Sample 4 can becontrolled by a horizontal electric field applied between the pixelelectrode and the common electrode.

Next, the transmittance of Sample 3 and Sample 4 were calculated. Thecalculation was performed using LCDMaster 3D (produced by SHINTECH,Inc.) in an FEM-Static mode. In the calculation, the size was 49.5 μmlong, 30 μm wide, and 4 μm deep (high), and the boundary condition was aperiodic boundary condition. The gate electrode was 200 nm thick, thegate insulating film was 400 nm thick, the pixel electrode was 0 nmthick, the signal line was 300 nm thick, an interlayer insulating filmwas 500 nm thick, and the common electrode was 100 nm thick in each ofSample 3 and Sample 4. A nitride insulating film between the pixelelectrode and the common electrode was 100 nm thick. The rubbingdirection of the liquid crystal was 90°, the twist angle was 0°, and thepretilt angle was 3°. Note that the pixel electrode was 0 nm thick inorder to reduce calculation load. The distribution of transmittance inthe case where −9 V was applied to the scan line, 0 V was applied to thecommon line, 6 V was applied to the signal line and the pixel electrodeof the left pixel, and 0 V was applied to the signal line and the pixelelectrode of the right pixel under the above conditions was evaluated.

The distribution of transmittance is expressed by grayscalc; a whiterregion has higher transmittance. FIG. 18C shows the distribution oftransmittance of Sample 3 and FIG. 18D shows the distribution oftransmittance of Sample 4.

In each of Sample 3 and Sample 4, white display and black display wereobserved on the left pixel and on the right pixel, respectively. In theblack display of Sample 3, a region with high transmittance (lightleakage) was partly observed. In contrast, in the black display ofSample 4, a region with high transmittance was not observed in theentire pixel. Since the drain electrode 263 has an L shape and overlapswith the end portion of the pixel electrode 249 in Sample 4, an electricfield between the scan line and the pixel electrode is less likely to begenerated in Sample 4 than in Sample 3, and light leakage in blackdisplay is reduced.

Thus, Sample 4 is an effective structure for a liquid crystal displaydevice with high contrast.

This application is based on Japanese Patent Application serial no.2013-177345 filed with Japan Patent Office on Aug. 28, 2013, andJapanese Patent Application serial no. 2014-047301 filed with JapanPatent Office on Mar. 11, 2014, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A liquid crystal display device comprising a pixel, the pixel comprising: a semiconductor film comprising a channel formation region, over a scan line with a first insulating film therebetween; a pixel electrode comprising a region over and in contact with the first insulating film; a conductive film electrically connected to the semiconductor film; and a signal line electrically connected to the semiconductor film, a second insulating film over the conductive film and the signal line; and a common electrode comprising an opening and a region over and in contact with the second insulating film, wherein the common electrode does not overlap with the channel formation region and comprises a region overlapping with the signal line, and a region overlapping with the pixel electrode, wherein the opening comprises a region not overlapping with the pixel electrode, wherein the opening comprises a first region extending along a first direction and a second region connected to the first region and extending along a second direction intersecting with the first direction in a plane view, wherein an entire region of the semiconductor film overlaps with the scan line, wherein the conductive film comprises a first region over and in contact with the semiconductor film, a second region over and in contact with the pixel electrode, and a third region between the first region and the second region, wherein the second region of the conductive film extends along a third direction parallel to or substantially parallel to the scan line, and wherein the third region of the conductive film extends along a fourth direction intersecting with the third direction.
 2. A liquid crystal display device comprising a pixel, the pixel comprising: a semiconductor film comprising a channel formation region, over a scan line with a first insulating film therebetween; a pixel electrode comprising a region over and in contact with the first insulating film; a conductive film electrically connected to the semiconductor film; and a signal line electrically connected to the semiconductor film, a second insulating film over the conductive film and the signal line; and a common electrode comprising an opening and a region over and in contact with the second insulating film, wherein the common electrode does not overlap with the channel formation region and comprises a region overlapping with the signal line, and a region overlapping with the pixel electrode, wherein the opening comprises a region not overlapping with the pixel electrode, wherein the opening has a shape comprising a bent portion in a plane view, wherein an entire region of the semiconductor film overlaps with the scan line, wherein the conductive film comprises a first region over and in contact with the semiconductor film, a second region over and in contact with the pixel electrode, and a third region between the first region and the second region, wherein the second region of the conductive film extends along a third direction parallel to or substantially parallel to the scan line, and wherein the third region of the conductive film extends along a fourth direction intersecting with the third direction.
 3. A liquid crystal display device comprising a pixel, the pixel comprising: a semiconductor film comprising a channel formation region, over a scan line with a first insulating film therebetween; a pixel electrode comprising a region over and in contact with the first insulating film; a conductive film electrically connected to the semiconductor film; and a signal line electrically connected to the semiconductor film, a second insulating film over the conductive film and the signal line; and a common electrode comprising an opening and a region over and in contact with the second insulating film, wherein the common electrode does not overlap with the channel formation region and comprises a region overlapping with the signal line, and a region overlapping with the pixel electrode, wherein the opening comprises a first region extending along a first direction and a second region extending along a second direction intersecting with the first direction, wherein the opening comprises a portion in which the first region is connected to the second region in a plane view, wherein an entire region of the semiconductor film overlaps with the scan line, wherein the conductive film comprises a first region over and in contact with the semiconductor film, a second region over and in contact with the pixel electrode, and a third region between the first region and the second region, wherein the second region of the conductive film extends along a third direction parallel to or substantially parallel to the scan line, and wherein the third region of the conductive film extends along a fourth direction intersecting with the third direction. 